Note: the `__update_widget()` method, while it doesn't have any visible delay
has the potential for improvement. I didn't do it because it felt like
premature optimization.
MoveGamePopUp: update it to use RareGame and it's signals
RareGame: Add `install_path` attribute and change `needs_verification` setter
Now both setters will update the local `igame` attribute and save it too.
MoveWorker: Update it to use RareGame.
Other changes include moving "same-drive" moving into the worker and using
`os.path` methods instead of `PathLib`
SteamGrades: Remove worker, it is implemented in RareGame now.
instead of adding them again to the layout. Move game should still
switch to RareGame, similar to the way it works for verifing.
Also add non-functional "Import Game" button
