cocoa_common: autohide dock/menubar on fs only if on same screen

Autohide the menubar and/or dock only if they are present in the screen the
player is going to go fullscreen into. I thought the GUI would handle this for
me when I switched 0057aa476 but lack of hardware to test made me embarass
myself yet again.

I reimplemented this feature with nicer code and behaviour. The code checks
separately wether to hide menubar and dock separatly, while the old code used
a single check possibly hiding stuff without need.

Added the key checks as a some category additions to NSScreen for readability.