Hide bar when Flash is fullscreen

I just started using Qtile tonight so please I need your feedback on this PR even if it is small. In particular, I'd like to know what happen behind the scenes.

Setting: I am currently using Qtile as window manager for KDE.
Problem: in Firefox, when I set a flash video fullscreen, the bar remains visible and hide the bottom part of the video (very annoying when there are subtitles)

I have fixed that by hiding the bar when the state _NET_WM_STATE_FULLSCREEN is chenged. However, I am not sure if I've put the snippet of code at the right place. Also, I don't understand why the bar become visible again I quit the fullscreen mode (this is the desired behavior but since I have added no code to make it visible again, I'm a bit puzzled).

Thanks for your feedback.