wayland: Set a more correct xdg_surface application id

The "app_id" of a xdg_surface should be the ID that can potentially be
used to get the DBUS name or the .desktop file.

For GtkApplication programs this is often the ID passed when creating the
GtkApplication object, so when available lets use that.

As fallbacks, first try g_get_prgname as it often corresponds to the
basename part of the .dektop file for non-GtkApplication programs.
Otherwise use gdk_get_program_class, even though that string usually
doesn't conform to the expectations of xdg_surface.set_application_id.

https://bugzilla.gnome.org/show_bug.cgi?id=746435