keyboard: Destroy the keyboard actor when disabled or after touch inactivity

When the keyboard is disabled, we were keeping the actor around, although it wasn't used and anyways re-created once enabled.

So, just call the _destroyKeyboard() now, after fixing it in order to make sure the keyboard works again once re-activated.

It's not needed to do this after the layout manager tween is completed in this case, because we request for an immediate hide (otherwise we might add signals or a callback parameter to the methods, but it looks unnecessary to me).