Allow showing the fallback app menu with a menu button
in the header bar. Applications have to explicitly enable
this by calling gtk_header_bar_set_show_fallback_app_menu.
Add a boolean property that controls whether a window close button
will be shown in the header bar or not. Doing this in the toolkit
will ensure consistency of the visual apperance.
https://bugzilla.gnome.org/show_bug.cgi?id=702971
