cocoa_common: prevent fullscreen window to black out other screens
cocoa_common: fix native-fs state saving

`opts->fs = VO_FALSE` was forgotten
cocoa_common: refactor fullscreen code for readability

Break up the code into several methods and reduce code duplication. Also add
comments to make the intention of all this clearer.
cocoa_common: change dock to autohide (as opposed to hide)