some deep changes to xml++ in which we retain a C-level xmlDocPtr as a member of...

some deep changes to xml++ in which we retain a C-level xmlDocPtr as a member of an XMLTree objects. this allows us to do repeated XPATH searches (as in the midnam parser of libmidi++) without constantly rewriting an entire tree into memory to recreate a new xmlDocPtr with which we can search. Since XMLTree objects don't typically stay around for very long, just when serializing to/from disk, this is not anticipated to have much (if any) impact on memory consumption

git-svn-id: svn://localhost/ardour2/branches/3.0@11733 d708f5d6-7413-0410-9779-e7cbd77b26cf