player: set PulseAudio stream title to window title

Set the PulseAudio stream title, just like the VO window title is set.
Refactor update_vo_window_title() so that we can use it for AOs too.

The ao_pulse.c bit is stolen from MPlayer.
ad_spdif: change API usage so that it works on Libav

Apparently we were using FFmpeg-specific APIs. I have no idea whether
this code is correct on both FFmpeg and Libav (no examples, bad
doxygen... why do they even complaint aht people are using their APIs
incorrectly?), but it appears to work on FFmpeg. That was also the case
before commit ebc4ccb though, where it used internal libavformat
symbols.

Untested on Libav, Travis will tell us.