substantive changes to the logic and safety for naming of (audio/MIDI) sources, especially when created via import
fix crash recovery: add new constructors to SndFileSource, AudioFileSource, add a new SourceFactory method and finally tweak
AudioDiskstream::use_pending_capture_data() to create both the required whole-file and the in-playlist regions
merge with master.

Manually resolved conflicts in import.cc and session.cc