Don't store the sender/receiver twice.

Not as bad as it looks, due to implicit sharing of QString's, but still
unnecessary. Instead use a bool to decide between sender and receiver. This
is of course global and not something that needs to be stored per item, but
it's much easier this way (and less risky to change for now). Since we
still have a few bits left in the ItemPrivate memory layout it doesn't cost
us anything.