Q3DSSceneElement::setCurrentSlideIndex sets index incorrectly

Incoming index is incremented by one for no apparent reason.

As this is a setter function, following sequence meant to increment slide by one works incorrectly: setCurrentSlideIndex(currentSlideIndex() + 1));

Not sure if changing this breaks any existing content, but the current functionality is not consistent.

edit: SceneElement API indexing still has different slide indexing compared to Q3DSPresentation. This is confusing when receiving slideEntered/Exited notifications from both SceneElement and Presentation, as numbers are off by one.

I hesitate changing the indexing of signaling from Q3DSPresentation (signals might be used internally in Runtime), and rather change SceneElement API to follow convention where slide 1 corresponds to index 1 (unlike in qt3d where first slide was index zero).