Add QJSValue::hasProperty() and hasOwnProperty() functions

Add QJSValue::hasProperty() and hasOwnProperty() functions

These functions provide a way of querying whether a property exists,
without relying on the QJSValue invalid type (which will be removed).

Task-number: QTBUG-23604
Change-Id: I2efd53a1e54cc202ecc022d12730b2775384cf53
Reviewed-by: Simon Hausmann <simon.hausmann@nokia.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@nokia.com>