Add QJSValue::callWithInstance() function

Add QJSValue::callWithInstance() function

With the deprecated call() overload, it was confusing what the first
argument was (the this-object or an actual argument passed to the
function). Introduce a dedicated function for the
"explicit this-object" case. This makes code more readable, and
eliminates the need to pass a "dummy" this-object to call() in the
quite common case where you don't care about the this-object.

Task-number: QTBUG-23604
Change-Id: I18f8be6592a848436351516bea266fc7e9195777
Reviewed-by: Simon Hausmann <simon.hausmann@nokia.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@nokia.com>