keyboard: Make sure to destroy the key's BoxPointer

When the keyboard is destroyed, we destroy the keyboard actor, but the
keyboard's menu isn't part of the key itself, so it's never tracked.

The menus are actually tracked actors, so they slow down the layout
manager's code to rebuild regions and other things. Keeping this list
small is a good idea.

To prevent leaking menus, destroy the menu when the key is destroyed.

https://bugzilla.gnome.org/show_bug.cgi?id=736999