[BUG] WindowManager focus_next() Reverse Wrapping

[lfgberg]

Describe the bug
When using focus_next() on a WindowManager with a negative step (ex. step=-1), a crash occurs instead of wrapping around to the next window as seen with a positive step

To Reproduce
Steps to reproduce the behavior:

1. Instantiate a WindowManager
2. Use .focus_next(step=-1) multiple times
3. Observe the crash.

Expected behavior
The WM should wrap and have the bottom window focused instead of crashing.

Possible solution
Opening a PR rn.

[lfgberg]

See #162