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.
osx: cocoa_common: use default wakeup period

Now that Cocoa's input handling is done on a separate thread from the playloop
it is ridicolously simple to have longer asynchronous sleeps when paused.
macosx_events: send all queued events

Seeking feels a little faster or it may be self suggestion. There's really no
reason to just send only one event at a time.
osx: implement media keys

Media keys are pretty handy if you use mpv as a music player (yes I'm one of
those people that do).

These are the bindings (which lead to the same behaviour as iTunes):
  * NX_KEYTYPE_PLAY   -> MP_KEY_PLAY
  * NX_KEYTYPE_FAST   -> MP_KEY_NEXT
  * NX_KEYTYPE_REWIND -> MP_KEY_PREV

I just handled these ones as the volume one would be pretty invasive. I could
maybe change it to increase the application's volume instead of system volume
only when mpv is frontmost (iTunes does this), but some users would probably
hate it.