LibGUI: Clear selection if right-clicking item that isn't selected

If we're right-clicking on an item that isn't currently selected, clear
the selection first.

Fixes #3665

Libraries/LibGUI/AbstractView.h

@@ -52,6 +52,7 @@ public:
         Set,
         Shift,
         Ctrl,
+        ClearIfNotSelected
     };
 
     virtual void move_cursor(CursorMovement, SelectionUpdate) { }