Avoid double app menu fallback

Avoid double app menu fallback

Both GtkApplicationWindow and GtkHeaderBar listen for changes
of the gtk-shell-shows-app-menu setting, so they need to somehow
coordinate who is going to take action and show a fallback.
We prefer the menu button in the title over the menubar, so
let GtkApplicationWindow opt out if it finds that the header bar
has been configured to show window controls.