Add QJSValue::call() overload

Add QJSValue::call() overload

This overload takes only an argument list, not a this-object, since
that is a very common way of calling stand-alone ("non-member")
functions. Now there is no longer a need to pass a dummy value for
the this-object.

Task-number: QTBUG-23604
Change-Id: Iae952d91fce5bcaa62a05b9978c15f32802da90a
Reviewed-by: Simon Hausmann <simon.hausmann@nokia.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@nokia.com>