UI: Set sidebar icon size to 16px

Symbolic icons, such as the ones used in the sidebar of the settings
dialog, are usually made to work best at 16x16.

UI/forms/OBSBasicSettings.ui

@@ -44,8 +44,8 @@
        </property>
        <property name="iconSize">
         <size>
-         <width>32</width>
-         <height>32</height>
+         <width>16</width>
+         <height>16</height>
         </size>
        </property>
        <property name="currentRow">