A Gtk.Application subclass to handle the application's initialization
and integration with the GNOME stack. It implements the do_startup and
do_activate methods and is responsible for the application's menus,
icons, title and lifetime.
As discussed in #5, this enables better integration with the GNOME stack
through Meson's GNOME module. This will allow us to use GSchemas,
AppData and other GNOME technologies much easier. It will also simplify
building Flatpaks of Piper.
