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.