cleanup up cleanup at session destruction; clarify the meaning of 3 signals (DropRefe...

cleanup up cleanup at session destruction; clarify the meaning of 3 signals (DropReferences & Destroyed in libardour ; CatchDeletion in the GTK UI); clarify ownership of objects (session no longer pays attention to DropReferences for objects that it is considered to own, such as routes, sources, etc); fix up MIDI parsing and a couple of other places by correcting syntax for return of values from a boost::signals2::signal (possible danger elsewhere to be checked)

git-svn-id: svn://localhost/ardour2/branches/3.0@6389 d708f5d6-7413-0410-9779-e7cbd77b26cf