cocoa: add --app-style option

This option allows changing the NSApplicationActivationPolicy used
by the cocoa application, which affects things like whether or not
a dock icon is displayed. Solves issue #635

In theory, this option can be somewhat cross-platform, e.g. on
Windows controlling whether or not a process bar button appears.
(But, as no platform besides OSX currently reacts to it, the
option is guarded by HAVE_COCOA for now.)
cocoa: change behavior of --app-style=auto

This changes some default behavior of mpv.
auto now selects --app-style=normal when launched as a bundle,
and --app-style=accessory when launched from the command line.
This affects whether or not a dock icon is displayed, and some
OSX fullscreen behavior with respect to other applications.