A base class for Adwaita applications.
`AdwApplication` handles library initialization by calling [func@init] in the default [signal@Gio.Application:
AdwApplication:startup
] signal handler, in turn chaining up as required by [class@Gtk.Application]. Therefore, any subclass of
`AdwApplication` should always chain up its `startup` handler before using any Adwaita or GTK API.
`AdwApplication` will automatically load stylesheets located in the application's resource base path (see [ method@Gio.Application.set_resource_base_path], if they're present.
They can be used to add custom styles to the application, as follows: