Use standard widgetry for app menu

[bragefuglseth]

The app menu appears to be using non-standard widgetry. The menu items have centered text and are taller than usual:

Instead of this, a GtkMenuModel should be attached to the button. This is the correct way to do it, so most other GNOME apps have example implementations that can be followed.

[giantpinkrobots]

I have implemented a new menu using Gio.Menu. It should look more in-line with other GNOME apps now.

Thanks for the heads up.