Don't call receivers from disconnectNotify

A pending documentation update is going to explain that you can't call
into QObject functions from disconnectNotify, or it may deadlock
on the signalSlot mutex.

Task-number: QTBUG-34829
Change-Id: I57e7b4e457d813d4255c8cdbe36228b2e0f45aaa
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>