Fix crash on signal after deregistration

Implemented a remove method in SignalHandler that allows
us to remove and disconnect an object from SignalHandler
w/o decrementing the connection counter until it hits
zero or deleting the object
That same functionality was used to remove an object from
internal lists when receiving a destroyed signal from an
object. In case of deregistering an object we haven't
received a destoryed signal but simulated reception of that
signal and so that code was not called in that case.

Change-Id: Ie20cf628a2de028375f5d29f913682e25ebf8d44
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>