Set a special window property for windows with menus

* hildon/hildon-window-private.h: Add private
hildon_window_set_menu_flag()
* hildon/hildon-window.c: (+do_set_has_menu),
(+hildon_window_set_menu_flag): set
"_HILDON_WM_WINDOW_MENU_INDICATOR" to 1 in the window
or remove the property.
(hildon_window_set_main_menu),
(hildon_window_set_app_menu): call above methods to set or remove
the property when a menu is added or removed from the window.

Partially fixes: NB#137814 (Indicator for view menu in app title area)