Replaces decoder_data() and others.

Replaces decoder_initialized().

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

This commit adds the basic infrastructure for reporting bugs from
DecoderAPI.cxx via DecoderThread.cxx to DecoderControl.

Reduce header dependencies and allow it to be nullptr to disable it.

Must hold the Mutex to signal the Cond object safely.

When playing a CUE track, the player thread waited for the decoder to
become ready, and then sent a SEEK command to the beginning of the CUE
track.  If that is near the start of the song file, and the track is
short enough, the decoder could have finished decoding already at that
point, and seeking fails.

This commit makes this initial seek more robust: instead of letting
the player thread deal with the difficult timings, let the decoder API
emulate a SEEK command, and return it to the decoder plugin, as soon
as the plugin finishes its initialization.