Rearranges the right-mouse click popup menus for the world editor and gui editors to a) be organized more logically and b) be more flexible.

This also fixes some insecure behavior relying on %this value eval'ing, which has also been modified to be better.

Also fixes up some old calls for getting menubar menus by internal name, which is no longer supported, instead using the findMenu function call.

Engine/source/gui/editor/popupMenu.h

@@ -137,6 +137,9 @@ public:
    /// Returns the number of items in the menu.
    U32 getItemCount();
 
+   ///Clears all items
+   void clearItems();
+
    //-----------------------------------------------------------------------------
    /// Displays this menu as a popup menu and blocks until the user has selected
    /// an item.