QItemDelegate - add a more comprehensive test

QItemDelegate - add a more comprehensive test

This is a bigger test after this change:
c3e1abad4e141e6e9d876e5cff194c473a2654eb

Beside the test of behavior it has the class
FastEditItemView which could be useful in the future.

QTestEventLoop::instance().enterLoop(1) does not perform well.
(IE it makes CI slower). My class could be used to extensions.
(or maybe even to remove a few of these calls)

Change-Id: I4f1460873cd07ddc482d5cfe462b59c47ebb189f
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>