UI: Fix minor stylesheet issues

This does a few small things
-Moves buttons down 20px to the same height as the list boxes
-Adds a QFrame around scrollArea for mixer list.

Q: Why was this done?
A: When you go to style the mixer list in regards to adding a border,
shadow, or glow, it needs to be done on the QFrame. If you do it on the
scrollArea itself, the scrollbars will overlap the bottom of the border,
causing the border to look cut-off. Additionally, the other two sources
and scenes list widgets already had frames, so they did not have this
problem.

obs/forms/OBSBasic.ui

@@ -30,6 +30,9 @@
    <iconset resource="obs.qrc">
     <normaloff>:/res/images/obs.png</normaloff>:/res/images/obs.png</iconset>
   </property>
+  <property name="styleSheet">
+   <string notr="true"/>
+  </property>
   <widget class="QWidget" name="centralwidget">
    <layout class="QVBoxLayout" name="verticalLayout">
     <item>
@@ -326,49 +329,82 @@
            </widget>
           </item>
           <item>
-           <widget class="VScrollArea" name="scrollArea">
-            <property name="verticalScrollBarPolicy">
-             <enum>Qt::ScrollBarAlwaysOn</enum>
+           <widget class="QFrame" name="frame_4">
+            <property name="sizePolicy">
+             <sizepolicy hsizetype="Preferred" vsizetype="Expanding">
+              <horstretch>0</horstretch>
+              <verstretch>0</verstretch>
+             </sizepolicy>
             </property>
-            <property name="horizontalScrollBarPolicy">
-             <enum>Qt::ScrollBarAlwaysOff</enum>
+            <property name="frameShape">
+             <enum>QFrame::StyledPanel</enum>
             </property>
-            <property name="widgetResizable">
-             <bool>true</bool>
+            <property name="frameShadow">
+             <enum>QFrame::Raised</enum>
             </property>
-            <widget class="QWidget" name="volumeWidgets">
-             <property name="geometry">
-              <rect>
-               <x>0</x>
-               <y>0</y>
-               <width>216</width>
-               <height>16</height>
-              </rect>
+            <layout class="QVBoxLayout" name="verticalLayout_7">
+             <property name="spacing">
+              <number>0</number>
+             </property>
+             <property name="leftMargin">
+              <number>0</number>
+             </property>
+             <property name="topMargin">
+              <number>0</number>
+             </property>
+             <property name="rightMargin">
+              <number>0</number>
              </property>
-             <property name="sizePolicy">
-              <sizepolicy hsizetype="Preferred" vsizetype="Maximum">
-               <horstretch>0</horstretch>
-               <verstretch>0</verstretch>
-              </sizepolicy>
+             <property name="bottomMargin">
+              <number>0</number>
              </property>
-             <layout class="QVBoxLayout" name="verticalLayout_6">
-              <property name="spacing">
-               <number>0</number>
-              </property>
-              <property name="leftMargin">
-               <number>0</number>
-              </property>
-              <property name="topMargin">
-               <number>0</number>
-              </property>
-              <property name="rightMargin">
-               <number>0</number>
-              </property>
-              <property name="bottomMargin">
-               <number>0</number>
-              </property>
-             </layout>
-            </widget>
+             <item>
+              <widget class="VScrollArea" name="scrollArea">
+               <property name="verticalScrollBarPolicy">
+                <enum>Qt::ScrollBarAlwaysOn</enum>
+               </property>
+               <property name="horizontalScrollBarPolicy">
+                <enum>Qt::ScrollBarAlwaysOff</enum>
+               </property>
+               <property name="widgetResizable">
+                <bool>true</bool>
+               </property>
+               <widget class="QWidget" name="volumeWidgets">
+                <property name="geometry">
+                 <rect>
+                  <x>0</x>
+                  <y>0</y>
+                  <width>231</width>
+                  <height>16</height>
+                 </rect>
+                </property>
+                <property name="sizePolicy">
+                 <sizepolicy hsizetype="Preferred" vsizetype="Maximum">
+                  <horstretch>0</horstretch>
+                  <verstretch>0</verstretch>
+                 </sizepolicy>
+                </property>
+                <layout class="QVBoxLayout" name="verticalLayout_6">
+                 <property name="spacing">
+                  <number>0</number>
+                 </property>
+                 <property name="leftMargin">
+                  <number>0</number>
+                 </property>
+                 <property name="topMargin">
+                  <number>0</number>
+                 </property>
+                 <property name="rightMargin">
+                  <number>0</number>
+                 </property>
+                 <property name="bottomMargin">
+                  <number>0</number>
+                 </property>
+                </layout>
+               </widget>
+              </widget>
+             </item>
+            </layout>
            </widget>
           </item>
          </layout>
@@ -396,7 +432,7 @@
            <number>0</number>
           </property>
           <property name="topMargin">
-           <number>0</number>
+           <number>20</number>
           </property>
           <property name="rightMargin">
            <number>0</number>
@@ -455,7 +491,7 @@
      <x>0</x>
      <y>0</y>
      <width>927</width>
-     <height>26</height>
+     <height>21</height>
     </rect>
    </property>
    <widget class="QMenu" name="menu_File">