osx_common: remove outdated ifdeffery

It was definining keycodes not defined in OS X < 10.5.
osx: create macosx_events to deal with keyDown events

On OSX with Cocoa enabled keyDown events are now handled with
addLocalMonitorForEventsMatchingMask:handler:. This allows to respond to
events even when there is no VO initialized but the GUI is focused.