Fix dirname() being called twice.  Closes: #316.
sid: Handle line lengths up to 2048 in song-length database.  Closes: #326.
mpg123: Use standard pause/seek/stop implementation.
psf: Remove custom seek/pause/stop functions.
psf: Don't send obsolete audio data after seeking.
xsf: Remove custom seek/pause/stop functions.
Remove ALSA backend for amidi-plug.  This is the only non-PCM decoder, and dropping it will allow greatly simplifying the codebase.
amidi-plug: Remove module-loading code.  FluidSynth backend is hardcoded now.
amidi-plug: Remove unused code.
amidi-plug: Pause is handled transparently by core now.
amidi-plug: Rewrite play loop to run in a single thread.  Simplify timing calculations.
amidi-plug: Simplify the plethora of backend init/reset functions.
Fix typo (amidiplug vs. amidi-plug).
Remove last occurences of obsolete InputPlugin API.
Remove InputPlayback::set_pb_ready() calls.
Remove obsolete parameters to InputPlugin::play() that most plugins were ignoring anyway.
Change InputPlugin::update_song_tuple() to match core.
Read metadata from the AVStream object as well as the AVFormatContext.  Closes: #329.
mpris2: Run gdbus-codegen from the Makefile.
Update for interface API changes in core.