Introduce LFIDBearer, a Class for menu bearing UI that offers IDs to menus

Move menu code from scroll list into there
Separate out interface for getting IDs

No longer bother with the focus manager, for menus where not necessary,
this ensures that if focus suddenly changes, it won't break menu UX flow.

Clean up all the static functions by using new static class functions

indra/newview/llviewermenu.h

@@ -142,8 +142,6 @@ bool handle_go_to();
 // Export to XML or Collada
 void handle_export_selected( void * );
 
-const LLUUID get_focused_list_id_selected();
-
 class LLViewerMenuHolderGL : public LLMenuHolderGL
 {
 public: