Fix resumemusic/musicfadein not working

It seems like they were unfinished. This commit makes them properly
work.

When a track is stopped with stopmusic() or musicfadeout(),
resumemusic() will resume from where the track stopped. musicfadein()
does the same but does it with a gradual fade instead of suddenly
playing it at full volume.

I changed several interfaces around for this. First, setting currentsong
to -1 when music is stopped is handled in the hook callback that gets
called by SDL_mixer whenever the music stops. Otherwise, it'd be
problematic if currentsong was set to -1 when the song starts fading out
instead of when the song actually ends.

Also, music.play() has a few optional arguments now, to reduce the
copying-and-pasting of music code.

Lastly, we have to roll our own tracker of music length by using
SDL_GetPerformanceCounter(), because there's no way to get the music
position if a song fades out. (We could implicitly keep the music
position if we abruptly stopped the song using Mix_PauseMusic(), and
resume it using Mix_ResumeMusic(), but ignoring the fact that those two
functions are also used on the unfocus-pause (which, as it turns out, is
basically a non-issue because the unfocus-pause can use some other
functions), there's no equivalent for fading out, i.e. there's no
"fade out and pause when it fully fades out" function in SDL_mixer.) And
then we have to account for the unfocus-pause in our manual tracker.

Other than that, these commands are now fully functional.

desktop_version/src/KeyPoll.h

@@ -76,6 +76,8 @@ public:
 
 	bool linealreadyemptykludge;
 
+	Uint64 pauseStart;
+
 private:
 	std::map<SDL_JoystickID, SDL_GameController*> controllers;
 	std::map<SDL_GameControllerButton, bool> buttonmap;