player: allow redrawing screen during seeks

If seeks take very long, it's better not to freeze up the display.

(This doesn't handle the case when decoding video frames is extremely
slow; just if hr-seek is used, or the demuxer is threaded and blocks on
network I/O.)
player: make quit exit immediately

Stopping playback canceled waiting, but executed the remainder of the
playloop, including things like executing pointless seeks.
player: some further playloop cleanups

Handle --term-playing-msg at a better place.

Move MPV_EVENT_TICK hack into a separate function. Also add some words
to the client API that you shouldn't use it. (But better leave breaking
it for later.)

Handle --frames and frame_step differently. Remove the mess from the
playloop, and do it after frame display. Give up on the weird semantics
for audio-only mode (they didn't make sense anyway), and adjust the
manpage accordingly.