make item, itemAtIndex, itemAt methods consistent

Repeater's itemAt takes an index, but "at" should always signify that it takes a pixel location. So we should deprecate itemAt and perhaps start using item(int idx) for such cases I think? Or, is that bad because of possible overloading anywhere else that we could have an item property or an item(something) method? Perhaps overloading isn't so bad.

Also, I think methods like itemAt(x, y) should be overloaded to take a QPointF, when possible. I want to try to make points available alongside (or gradually replacing) separate x and y properties in the future, because it can make QML code more succinct: one binding instead of two, or not having to drill down twice to get a point twice so that you can call .x on it for one parameter and .y for the second.