Introduce QItemDelegate::destroyEditor virtual invoked at editor close

Introduce QItemDelegate::destroyEditor virtual invoked at editor close

This provides a stronger mechanism e.g when inheriting QItemDelegate.

It makes some things much easier e.g avoid delete of an editor
and maybe only delete depending on what the editor says itself.

This introduces a new virtual function.

Task-number: QTBUG-2299

Change-Id: I8410f8199775987dbacffd99e4c354fdadcdd21f
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>