input: move files drag and drop to a new event.c file

event.c will be used to feed the input queue with 'global' events that don't
mesh well with the usual check_events path in mpv.
cocoa: refactor files drag and drop on the Dock icon

Use the newly added `mp_event_drop_files` core function instead of having
logic in the platform dependent code.
cocoa: handle files drag and drop on the player video view