Delay sending activation token

We currently set the view as activated right away in phoc_xdg_activation_v1_handle_request_activate. Delay this if the view isn't mapped. This makes sure we send the token later and should also give !393 (closed) a chance to do that at a later point.

This fixes the flicker when the splash screen hides for applications using xdg-activation (like GTK4 apps).

Helps phosh#646 but we need to fix the gtk_shell1 case for GTK3 to fully resolve this.