NSApp.terminate() is not a requirement to properly shut down a cocoa
App since it only calls exit() internally. though when not used the
cocoa termination events won't trigger, which we don't need. this
prevented us to exit with a proper exit code.
rework the whole termination logic to end up at one point where we can
return the exit code from the mpv_main function.
Fixes#7456
this event has several advantages, it unifies the mpv:// url handling,
the dropping of files on the App icon and opening via finder into one
event, and it also lets us remove the file open workaround.
we had to keep track of opened files via the command line because the
event was also triggered by passed files on the command line, leading
to redundant load events.
the new event doesn't trigger from files passed via the command line
anymore.
