'Called when the previous client in Layout shall be focused.
This method should:
- Return the previous client in Layout, if any.
- Return None if the previous client would be the last client.
- Not focus the client itself, this is done by caller.
Do not implement a full cycle here, because the Groups cycling relies
on returning None here if the end of Layout is hit,
such that Floating clients are included in cycle.
Parameters
win:
The currently focused client.'

@abstractmethod
def focus_previous(self, win):

    pass
