Adding visual feedback on valid drop elements when an element is being dragged

Just making sure you find the idea interesting before continuing.

1. Since the use-case for this is more specific than for "dragHoverClass" and potentially affects more elements on each drag action, the default might be to apply no class instead for performance reasons. I could change it to be that way if you prefer.

2. I also noticed that "dragEnterClass" is only removed, never added. Seems deprecated. I could remove it from the code and documentation too.