QQuickItemParticle: Only delete owned particles

QQuickItemParticle: Only delete owned particles

A common pattern for ItemParticle (as shown in the particleview.qml
example) is to use it to manage movement of a model's delegates. In such
a case item management is handled by the user code (which bridges the
needs of the Model and the View) and deleting the items in ItemParticle
quickly leads to crashes.

This change maintains (and improves, due to handling resets) the
deletion of delegates created by the ItemParticles, as shown in
delegates.qml. This example was expanded with a click to reset feature
so as to more clearly see the impact of resets.

Task-number: QTBUG-37486
Change-Id: I9de935034c11a7dd2abdd60e7b3bd42867dede9c
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>