Alert the user if a connection is made which causes

Alert the user if a connection is made which causes
feedback, and preserve the route graph in the state
that it was in before the feedback was introduced. The
intent being to simplify the code, reduce the number of
areas of code which must consider feedback, and fix a
few bugs.

git-svn-id: svn://localhost/ardour2/branches/3.0@10510 d708f5d6-7413-0410-9779-e7cbd77b26cf