Copies text, tags, and pixbufs between start and end (the order of
start and end doesn't matter) and inserts the copy at iter. Used
instead of simply getting/inserting text because it preserves images
and tags. If start and end are in a different buffer from this buffer,
the two buffers must share the same tag table.
Implemented via emissions of the insert-text and apply-tag signals, so
expect those.