UI: Fix Delete key not working on scenes/sources

This commit fixes two regressions introduced by the modular UI (commit
25bb8a444f):

 - the Delete key no longer works in the scene list
 - the Delete key no longer works working in the source list

This commit also enables the Delete key on sources that were selected by
clicking on them in the preview, which addresses Mantis Bug 576
(https://obsproject.com/mantis/view.php?id=576).

UI/forms/OBSBasic.ui

@@ -86,6 +86,7 @@
             <property name="contextMenuPolicy">
              <enum>Qt::CustomContextMenu</enum>
             </property>
+            <addaction name="actionRemoveSource"/>
            </widget>
           </item>
          </layout>
@@ -400,6 +401,7 @@
           <property name="defaultDropAction">
            <enum>Qt::TargetMoveAction</enum>
           </property>
+          <addaction name="actionRemoveScene"/>
          </widget>
         </item>
         <item>
@@ -531,6 +533,7 @@
           <property name="selectionMode">
            <enum>QAbstractItemView::ExtendedSelection</enum>
           </property>
+          <addaction name="actionRemoveSource"/>
          </widget>
         </item>
         <item>