audio: refactor mixer code and delete mixer.c

mixer.c didn't really deserve to be separate anymore, as half of its
contents were unnecessary glue code after recent changes. It also
created a weird split between audio.c and af.c due to the fact that
mixer.c could insert audio filters. With the code being in audio.c
directly, together with other code that unserts filters during runtime,
it will be possible to cleanup this code a bit and make it work like the
video filter code.

As part of this change, make the balance code work like the volume code,
and add an option to back the current balance value. Also, since the
balance semantics are unexpected for most users (panning between the
audio channels, instead of just changing the relative volume), and there
are some other volumes, formally deprecate both the old property and the
new option.

player/main.c

@@ -52,7 +52,6 @@
 
 #include "audio/decode/dec_audio.h"
 #include "audio/out/ao.h"
-#include "audio/mixer.h"
 #include "demux/demux.h"
 #include "stream/stream.h"
 #include "sub/osd.h"
@@ -360,7 +359,6 @@ struct MPContext *mp_create(void)
 
     mpctx->input = mp_input_init(mpctx->global);
     screenshot_init(mpctx);
-    mpctx->mixer = mixer_init(mpctx, mpctx->global);
     command_init(mpctx);
     init_libav(mpctx->global);
     mp_clients_init(mpctx);