Improve buffer timestamping

Improve buffer timestamping

Make buffer timestamps more accurate and, more importantly, actually
representative of the MIDI events timing.

Previously, buffers were only sent with timetamps aligned at a 10ms
boundary which was just wrong, now the buffer timestamp represents the
real time of the MIDI event.

Conveniently, the ALSA sequence API supports scheduling events in the
future so the sequencing infrastructure can be used to have the tick
delivered at the right time, avoiding any custom scheduling mechanism.

The sequencer queue is started on the first transition to PLAYING, and
the delay is also calculated when the pipeline goes into PLAYING.