boxpointer: Don't hide when we're already hidden

boxpointer: Don't hide when we're already hidden

You would think we would already do something like this, but apparently
lots of code was calling hide() without checking if the box pointer was
already visible, causing it to queue a full tween. The biggest win was
with ibusCandidatePopup.js, which called hide() on every DBus message.

This increases the performance for me to enter the overview by a tiny
bit. The remaining time is spent updating the frequent apps / all apps
display.

https://bugzilla.gnome.org/show_bug.cgi?id=712727