The ObjectPool class template provides parts of the functionality
of the global PluginManager object pool but is intented to be
used with smaller set objects, typically with same base type (e.g.
factories) only.

The ObjectPool takes ownership of add items if and only if the item does
not have a QObject parent.

Items owned by the Object pool are destructed when the pool is
destructed, the other items are taken care of by their QObject parent
according to the usual parent/child behavior.

Change-Id: I60886095c8b04eae017e1fb56774b1bf66dbefa1
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>