destruct qobject wrappers before sweeping the GC heap

destruct qobject wrappers before sweeping the GC heap

The wrappers emit a destroyed signal, and it's important
that the GC heap is in a well defined state when these signals
are emitted.

Change-Id: I423c4241b1e2fd3de727277d26bbe64f08862193
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>