Read and write QObject pointer properties in QML without registration.

Previously, accessing QObject pointer properties from QML would require
these types to be registered with qRegisterMetaType(), but this
shouldn't be necessary if we're able to read/write the property, because
the moc generates code that calls qRegisterMetaType in the static meta-call
implementation. So when resolving a property in the property cache and we
can't resolve, fall back to placing the static meta-call to register the
type, similar to what QMetaType::userType() does.

Change-Id: Ic8b00ed93a1e5e42cf7aaaf1c355e89557485c59
Reviewed-by: Christopher Adams <chris.adams@jollamobile.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@theqtcompany.com>