Don't start two drags.

Problem was that the SourceTreeView called setEnableMouseTracking( true ) which means that it will get more events than we want. We don't need mouse move events when no button is pressed anyway.

src/sourcetree/sourcetreeview.cpp

@@ -29,7 +29,6 @@ SourceTreeView::SourceTreeView( QWidget* parent )
     setDragDropMode( QAbstractItemView::DropOnly );
     setAcceptDrops( true );
     setDropIndicatorShown( false );
-    setMouseTracking( true );
     setAllColumnsShowFocus( false );
 
     setupMenus();