GtkApplication: avoid using stale timestamps

GtkApplication: avoid using stale timestamps

Avoid using a stale timestamp (from the last user interaction with the
application) when a message arrives from D-Bus requesting that a new
window be created.

In this case the most-correct thing that we can do is to use no
timestamp at all.

We modify gdk_x11_display_set_startup_notification_id() to allow a NULL
value to mean "reset everything" and then call this function
unconditionally on receipt of D-Bus activation requests.  The result
will be that a missing desktop-startup-id in the platform-data struct
will reset the timestamp.

Under their default configuration metacity and mutter will both map
windows presented with no timestamp in the foreground.  This could
result in false-positive, but there is very little we can do about that
without the original timestamp from the user event.

https://bugzilla.gnome.org/show_bug.cgi?id=752000