midway snapshot of work done on managing Region & Source lifetimes correctly. may...

midway snapshot of work done on managing Region & Source lifetimes correctly. may fix missing MIDI file bug ; save empty playlists because they may be referred to by the history file ; undo commands auto-delete when objects they refer to die (currently not commands built from XML deserialization); Sources now know how many regions are using them for something, meaning that we know if we can delete the files holding any data for the source

git-svn-id: svn://localhost/ardour2/branches/3.0@7291 d708f5d6-7413-0410-9779-e7cbd77b26cf