If a section or submenu item has a "action-namespace" attribute, the
action names of the created GtkModelMenuItems will be prefixed with that
namespace. Namespaces can be cascaded.
Drop the explicit passing of GActionGroup into the GtkMenu(Bar)
constructors and operate from the action context instead.
With GtkMenuItem implementing GtkActionable, this turns out to be pretty
easy (and most of the code can be removed from GtkModelMenuItem,
including the GActionObserver implementation).
Right now, when we create a GtkModelMenu for a GMenuModel, we listen to
changes to the menu's attach-widget to detect when a toplevel
GtkApplicationWindow becomes available to fetch actions from it.
This unfortunately breaks this simple code:
GtkWidget *application_window = gtk_application_window_new();
GtkWidget *menu_button = gtk_menu_button_new();
GMenuModel *menu_model = get_menu_model();
gtk_menu_button_set_menu_model(menu_button, menu_model);
gtk_container_add(GTK_CONTAINER(application_window), menu_button);
Since GtkMenuButton creates a GtkModelMenu and sets itself as its attach
widget before it's added to a hierarchy containing a
GtkApplicationWindow.
Fix the bug by simply listening for changes in the window hierarchy, and
creating the menu model when the attach widget is added to an
application window.
https://bugzilla.gnome.org/show_bug.cgi?id=679454
Put this in a separate file and substantially refactor it.
Move handling of submenu creation into gtkmodelmenuitem where it
belongs.
Improve our handling of when to show separators or not.
