fix evoral assignment operator (copy buffer)
fix midi source factory -- load_model() uses shared_from_this()
open external [read-only] midi-files on construction
take all tracks inside a midi-file into account when loading model

* fixes [region] length calculation,
* prepare for auditioning midi files (one track plays it all)
backend for auditioning midi-files:

* "downgrade" auditioner from AudioTrack to Track.
* add relevant methods from both AudioTrack and MidiTrack.
remove cruft
prepare GUI for auditioning midi files
reset midi synth (panic) when seeking or re-starting audition
tweak reasonable synth:

* prevent denormals / cut low volume
* reduce default amplitude of fundamental
prepare midi-audition synth plugin config, incl no plugin external synth