workspace: Update focus when a window is moved away from the current desktop
If a focussed window is to a new virtual desktop we need to update focus. It does not make sense for focus to remain on a now-invisible window. This patch re-uses the same path as when virtual desktop is changed.
(cherry picked from commit b96e6cf0)
Co-authored-by: David Edmundson kde@davidedmundson.co.uk