Pretty much like dd4709bb, BoxPointer's show() and hide()
functions will clash with Clutter.Actor's ones.

In addition to that, on a conceptual level, the current API
is not great, because calling boxPointer.hide() won't result
in boxPointer.actor.visible == false.

For these reasons, rename show() and hide() to open() and
close(). A compatibility layer will be added in a following
commit, warning about the usage of show() and hide().

!153