[Drag&Drop] Let CGUIControls be drop targets

Here is my first part of my Drag&Drop stuff.
It let's skinners add a <ondrop> action to all CGUIControls (excepts Containers like Panels.). This ondrop action will be executed when the user drags a listitem on it.

I also made a skin change, that shows the dragged items icon under the mouse. Doesn't look bad, but I guess a skinner could improve on that.