revealer: emit notify::child-revealed when animation stops

revealer: emit notify::child-revealed when animation stops

Depending of float rounding during target calculation, the size of the
GtkRevealer can be set to zero will the animation is not finished.
If the GtkRevealer is in a GtkPaned, it will be hidden and so the animation
will be stopped before it is finished.

In this case, force the emission of the child-revealed signal to let
client code know the animation is finished.

https://bugzilla.gnome.org/show_bug.cgi?id=765973