AppMenu: remove tweens before animating the actor visibility

When we show(), we need to make sure that the hiding animation
doesn't reach the end, otherwise we would hide the actor but
still have _visible = true.
We were relying on tweener overwriting to do this, but it
doesn't quite work, so better be explicit and do it ourselves.

https://bugzilla.gnome.org/show_bug.cgi?id=707814