GtkMenuTracker: add hidden-when='macos-menubar'

GtkMenuTracker: add hidden-when='macos-menubar'

Provide a mechanism for hiding the "Quit", "About" and "Preferences"
menu items from the normal places in a traditional menubar layout (in
the File and Edit menus) when the menu is being rendered in the Mac OS
menubar.

These items can already be found in the application menu.

With this feature, applications can now define a single menu to use in
all 'traditional' scenarios.

Use this new attribute in Bloatpad.

https://bugzilla.gnome.org/show_bug.cgi?id=741610