frontend: Move media control dynamic properties to ui file

This moves the dynamic properties for the media control buttons
from the cpp file to the ui file.

frontend/components/MediaControls.cpp

@@ -46,10 +46,6 @@ void MediaControls::OBSMediaPrevious(void *data, calldata_t *)
 MediaControls::MediaControls(QWidget *parent) : QWidget(parent), ui(new Ui::MediaControls)
 {
 	ui->setupUi(this);
-	ui->playPauseButton->setProperty("class", "icon-media-play");
-	ui->previousButton->setProperty("class", "icon-media-prev");
-	ui->nextButton->setProperty("class", "icon-media-next");
-	ui->stopButton->setProperty("class", "icon-media-stop");
 	setFocusPolicy(Qt::StrongFocus);
 
 	connect(&mediaTimer, &QTimer::timeout, this, &MediaControls::SetSliderPosition);

frontend/forms/source-toolbar/media-controls.ui

@@ -74,6 +74,9 @@
      <property name="flat">
       <bool>true</bool>
      </property>
+     <property name="class" stdset="0">
+      <string>icon-media-play</string>
+     </property>
     </widget>
    </item>
    <item>
@@ -112,6 +115,9 @@
      <property name="flat">
       <bool>true</bool>
      </property>
+     <property name="class" stdset="0">
+      <string>icon-media-prev</string>
+     </property>
     </widget>
    </item>
    <item>
@@ -150,6 +156,9 @@
      <property name="flat">
       <bool>true</bool>
      </property>
+     <property name="class" stdset="0">
+      <string>icon-media-stop</string>
+     </property>
     </widget>
    </item>
    <item>
@@ -188,6 +197,9 @@
      <property name="flat">
       <bool>true</bool>
      </property>
+     <property name="class" stdset="0">
+      <string>icon-media-next</string>
+     </property>
     </widget>
    </item>
    <item>