UI: Minimize transitions pane size on main window

Prevents it from taking up too much space from the other parts of the
main window.

obs/forms/OBSBasic.ui

@@ -600,6 +600,9 @@
               <property name="orientation">
                <enum>Qt::Horizontal</enum>
               </property>
+              <property name="sizeType">
+               <enum>QSizePolicy::Preferred</enum>
+              </property>
               <property name="sizeHint" stdset="0">
                <size>
                 <width>40</width>