Emit destruction signal before child contexts are destroyed.

Emit destruction signal before child contexts are destroyed.

Verbatim comment from bug report:

When using a QtObject inside an item, and then we call a function
in the Component.onDestruction handler of that item, we get a
crash. This happens because the QDeclarativeContextData engine
has been invalidated before reaching
QDeclarativeExpressionPrivate::evalFunction.

Change code to emit the onDestruction signal before the child
contexts are invalidated.

Task-number: QTBUG-22535
Change-Id: Ic4983ae5fdf104ae977189c21dc202c9b02bc2bc
Reviewed-by: Chris Adams <christopher.adams@nokia.com>