add begin/end undo/redo signals so that playlist can freeze/thaw itself around potent...

add begin/end undo/redo signals so that playlist can freeze/thaw itself around potentially NxM region property changes; clean up debug output; don't connect streamview to Playlist::ContentsChanged because there appears to be no need for it - we catch add/remove region, and region property changes are handled by RegionView

git-svn-id: svn://localhost/ardour2/branches/3.0@6702 d708f5d6-7413-0410-9779-e7cbd77b26cf