frontend: Re-set list spacing of source and scene tree to 0

This is needed due to QTBUG-106395. With spacing, drag-and-drop would
cause items to go to the bottom of the list if they are dropped in the
spacing.

Effectively reverts 5fa4ea44d0b364bf9be18816c2d9b35b96bab939.
See also #7321 and 860b309db86ac4e096df1d72b9cf8f99d60e8c56.

frontend/forms/OBSBasic.ui

@@ -1033,8 +1033,9 @@
           <property name="defaultDropAction">
            <enum>Qt::TargetMoveAction</enum>
           </property>
+          <!-- Spacing must be 0 due to QTBUG-106395 -->
           <property name="spacing">
-           <number>1</number>
+           <number>0</number>
           </property>
           <addaction name="actionRemoveScene"/>
          </widget>
@@ -1155,8 +1156,9 @@
           <property name="selectionMode">
            <enum>QAbstractItemView::ExtendedSelection</enum>
           </property>
+          <!-- Spacing must be 0 due to QTBUG-106395 -->
           <property name="spacing">
-           <number>1</number>
+           <number>0</number>
           </property>
           <addaction name="actionRemoveSource"/>
          </widget>