[Models/contacts-model] Use QSharedPointer rather than QWeakPointer
[Models/contacts-model] Use QSharedPointer rather than QWeakPointer

We already create QSharedPointers, but QWeakPointer wasn't happy with
getting QSharedPointer passed and stayed null, which resulted in empty
contact list.

Now the list should be back to normal again.

Reviewed-by: David Edmundson

KDE/ktp-common-internals,KDE/ktp-common-internals,KDE/ktp-common-internals