I suggested to additionally parse +- as add/remove items in
a multilink property in emails (#541941). Thanks for
implementing this in 0.4.2. However, I noticed that 
the traditional interpretation (i.e. overwriting all
previous values)
no longer works: <propname>=A is interpreted just
like <propname>=+A.

Attached is a patch that fixes this, because I think that
all three methods have their justification (besides, some
people might be used to the old method).

<propname>=A,+B,-C is treated like <propname>=A,B.