Actually implement opening the app menu

The last commit added support for the "appmenu" button in decorations,
but didn't actually implement it. Add a new MetaWindowMenuType parameter
to the show_window_menu () functions and use it to ask the compositor
to display the app menu when the new button is activated.

https://bugzilla.gnome.org/show_bug.cgi?id=730752

src/meta/common.h

@@ -387,6 +387,19 @@ struct _MetaButtonLayout
   gboolean right_buttons_has_spacer[MAX_BUTTONS_PER_CORNER];
 };
 
+/**
+ * MetaWindowMenuType:
+ * @META_WINDOW_MENU_WM: the window manager menu
+ * @META_WINDOW_MENU_APP: the (fallback) app menu
+ *
+ * Menu the compositor should display for a given window
+ */
+typedef enum
+{
+  META_WINDOW_MENU_WM,
+  META_WINDOW_MENU_APP
+} MetaWindowMenuType;
+
 /**
  * MetaFrameBorders:
  * @visible: inner visible portion of frame border