Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

Omitting it has the same effect as returning false unconditionally.

Eliminate input_stream_init() and input_stream_deinit().

Hide the definition from C code, to prepare the transition to C++.

Add GMutex, GCond attributes which will be used by callers to
conditionally wait on the stream.

Remove the (now-useless) plugin method buffer(), wait on GCond
instead.  Lock the input_stream before each method call.  Do the same
with the playlist plugins.

For Spotify tracks. Uses a spt URI, so with mpc you can play tracks
with e.g.,

  mpc add spt://spotify:track:5qENVY0YEdZ7fiuOax70x1
  mpc play

Uses the pcm_decoder_plugin for the output