Pass button_rect when opening window menu from button

When opening the window menu without an associated control - e.g.
by right-clicking the titlebar or by keyboard - using coordinates
for the menu position is appropriate. However when the menu is
associated with a window button, the expected behavior in the
shell can be implemented much easier with the full button geometry:
the menu will point to the center of the button's bottom edge
rather than align to the left/right side of the titlebar as it
does now, and the clickable area where a release event does not
dismiss the menu will match the actual clickable area in mutter.

So add an additional show_window_menu_for_rect() function and
use it when opening the menu from a button.

https://bugzilla.gnome.org/show_bug.cgi?id=731058

src/core/window.c

@@ -5251,6 +5251,15 @@ meta_window_show_menu (MetaWindow         *window,
   meta_compositor_show_window_menu (window->display->compositor, window, menu, x, y);
 }
 
+void
+meta_window_show_menu_for_rect (MetaWindow         *window,
+                                MetaWindowMenuType  menu,
+                                MetaRectangle      *rect)
+{
+  g_return_if_fail (!window->override_redirect);
+  meta_compositor_show_window_menu_for_rect (window->display->compositor, window, menu, rect);
+}
+
 void
 meta_window_shove_titlebar_onscreen (MetaWindow *window)
 {