Allow views to detect if they are being painted for drag and drop

[timboudreau]

Rationale: The default painting behavior of components which are being dragged is not always ideal:

• The only way (that I have found, anyway) for a View to receive drag events when the drag moves outside the view or window bounds is to call View.draggable().
• That has the side-effect of altering the painting behavior to paint the component twice, with one copy under the mouse cursor.
• There was (up until now) no way to detect whether a view was being painted because of a drag operation in progress (or the rebound on an unaccepted drag), or painting normally - or differentiate them if you expect to need normal repaints during the drag operation, but you do not want paint a second copy of your component at all (or want to paint something else) only for the drag-based invocations of View.paint()
• Getting rid of the drag image entirely still involves some non-intuitive steps * Use this new call to detect drag paints and return from the paint method without painting at all * Set a drag style of Background : Color::TRANSPARENT on the View at initialization (otherwise you'll drag a background-colored rectangle around the screen instead)

Ideally it might be useful to have one or both of

• A way for a View to signal don't paint a drag image of me at all - probably as something you can set on the style - but being able to paint an alterative rendering of the component is useful as well, OR
• A way for a View to lock itself into the set of recipients of mouse motion events until the next mouse up, without any implications about OS-level drag and drop

but this both solves the problem and is useful. I'll try to contribute an example of combining this with the undecorated window patches I submitted earlier to demonstrate the sort of thing it's good for, along the lines of this.