clutter/stage: Turn `get_key_focus()` into a proper getter
The method looks like a getter of the key-focus property
and is detected as such, but it returns the stage itself
when the property is NULL.
This has become problematic now that gjs optimizes property accesses by calling the corresponding setter:
To address this, add a new method that replicates the existing behavior, then change the existing method to always return the property value.