When a playlist is deleted and drops_references(), any
undo/redo StatefulDiffCommand referncing playlist invoke
Destructible::drop_references() of the Command.
This leads to command_death(). As opposed to UndoTransaction::clear()
the StatefulDiffCommand was not destroyed.
In case of playlists StatefulDiffCommand::_changes contains
PBD::SequenceProperty<std::list<boost::shared_ptr<Region> > >
and shared pointer reference of the playlist regions were kept
indefinitely.
This fixes the following scenario:
New session, import an file, delete the created track,
clean up unused sources (delete unused playlists)[, quit].
A reference to the imported region was kept, because of the
playlist's undo command (insert region). Yet the source file
was deleted.
PS. Most playlist changes are accompanied by GUI zoom/selection
MementoCommands. Those are currently never directly dropped.
command_death() leaves those in place.
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