remove cruft
lock source list when destroying it.
one less Conditional jump or move depends on uninitialised value(s)
sfdb's clock-states are not part of the session-state, use transient clock

fixes issue with 'show import dialog' marking session as modified.
ditto for clock in session properties dialog
fix bitslot already in use warning

in e45151b89c64 route.cc was changed to create internal sends
directly with role = Delivery::Aux; and not Delivery::Role (0).
This change was motivated to initialize the panner for Aux-sends
in the Delivery.

Role(0) was used to override bitslot numbering during initial
construction of the object when the state is loaded from XML after
construction.

This patch adds an explicit flag for that.
(The previous Role(0) approach only worked for Aux-Sends
but not Sends, anyway.)
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
fix crash -- midi tracker reset when seeking audio
attenuate master volume
Update Russian translation

Resync some terminology with proposed changes in the translation
of Mixbus (session, snapping, monitoring).
configurable midi audition synth