Change repeater item to handle model being deleted.

The repeater item previously stored a raw QObject pointer in
a variant. When this pointer was a dynamic list model element
that was deleted, the variant would continue to hold a stale
pointer. Change repeater to use a guard object to hold the model
when it is a QObject. Continue to use a variant to hold models
that are not based on QObject to maintain same semantics.

Change-Id: Ie100947132923803263c725e86efa68206382f12
Reviewed-by: Martin Jones <martin.jones@nokia.com>