focus method uses ctypes.windll on mac OS

[hannesdelbeke]

Since the focus.py module imports ctypes, this likely works only on Windows.

TODO

• Move the windows logic to the windows class.
• figure out how to split the logic to support multiple OS in the future
• test on Mac and Linux

relates to

• #12
• #70

[hannesdelbeke]

ctypes aparently might work on linux and mac, but not the way we are using it, so issue still stands
ctypes.win32 or ctypes.windll

we use ctypes.windll.user32 in bqt, is this supported on mac? and linux? if not we should refactor