Add a dedicated export method to MidiRegion

Add a dedicated export method to MidiRegion

To export a MIDI region to a file, the code used MidiRegion::clone()
since it takes care of creating a new file-backed source with the wanted
contents. Nevertheless, it had several side-effects:
- it created and registered a new region which is confusing to users
- it only exported notes that were in the region range, but didn't
  remove the region start offset from MIDI events, essentially producing
  a spurious silence at the beginning of the exported file (this is not
  a problem for region cloning because the newly created region is made
  aware of the offset and caters for it).

Add a dedicated code path for export, that uses the new offsetting
capabilities of MidiModel::write_section_to().