OSX: run native event loop in a separate thread

This commit is a followup on the previous one and uses a solution I like more
since it totally decouples the Cocoa code from mpv's core and tries to emulate
a generic Cocoa application's lifecycle as much as possible without fighting
the framework.

mpv's main is executed in a pthread while the main thread runs the native cocoa
event loop.

All of the thread safety is mainly accomplished with additional logic in
cocoa_common as to not increase complexity on the crossplatform parts of the
code.
macosx_application: add menu item for hide operation

Fixes #35
cocoa_common: add native OSX fullscreen support

This adds Mission Control fullscreen functionality to mpv. Since this doesn't
play well with many of mpv's features disable it by default. Users can activate
this feature by using `--native-fs` when starting mpv.

Fixes #34