Can pass a core to ardbg now. Fixed sometimes crash on saving history by

Can pass a core to ardbg now. Fixed sometimes crash on saving history by
creating a memory leak(?) that will go away with the transition of XMLNode* to
shared_ptr<>. A few bits toward restoring history from XML.

git-svn-id: svn://localhost/ardour2/branches/undo@779 d708f5d6-7413-0410-9779-e7cbd77b26cf