new region constructor for use by audiofilters (almost does a pure copy of the region...

new region constructor for use by audiofilters (almost does a pure copy of the region, but sets start & sync_position appropriately, and uses new sources); fix up reverse bugs introduced in the last 36hrs; fix up setting of sync_position after split and so forth

git-svn-id: svn://localhost/ardour2/branches/2.0-ongoing@4140 d708f5d6-7413-0410-9779-e7cbd77b26cf