clutter: Bypass priv->has_key_focus when unsetting focus from self

This is conditionally toggled by grabs on the current key focus depending on whether the current key focus actor would receive events according to the grab or not. Which means it's no longer a reliable method for an actor to know it does have focus, without asking the stage about it.

Avoid this check and ask the stage for the key focus, in order to make key focus actors able to unset themselves despite the presence of grabs.