Presently, Gtk will only send a startup notification completion message
for the first window that is shown.  This is not good for the case of
GtkApplication, where we are expected to participate in
startup-notification for all windows.

We have avoided this problem by manually emitting the startup complete
message from after_emit in GtkApplication.

Unfortunately, this causes problems for windows that are shown with a
delay.  It is also a dirty hack.

The reason for the original behaviour is simple: there is a static
boolean in gtkwindow.c which controls it.  We remove this.

Instead, clear the startup notification ID stored in GDK when sending
the completion message.  GtkApplication will re-set this the next time
an event comes in which needs startup-notification handling.  In the
non-GtkApplication case, newly shown windows will still not send the
message, since the cookie will have been cleared.

Finally, we remove the hack from GtkApplication's after_e...