[Dev] Drag and Drop cursor feedback - in transition

[Donn Denman]

The cursor feedback being given in Drag & Drop is in transition right  
now - it should be settled by mid-week.

There has been some regression on Mac and Linux that I'll correct  
soon -- the dragging still works OK but the feedback you get from the  
cursor about whether you are doing a move versus a copy is wrong on  
those platforms, and there are some drawing artifacts.

I like the way it works on PC - now we can show different cursors  
when dragging over different individual items.  I updated the Sidebar  
with my stab at some reasonable behavior, but  we need to review this  
with Mimi and Alec.  E.g I hope we can do a little "trash" cursor for  
the Trash.

Here's what the Sidebar currently does on PC:
  * regular collections - the "copy" cursor
  * Trash collection - the "move" cursor
  * Views (or and non-collection)  - the "no" cursor
  * white space - the "no" cursor
  * a collection that already contains all the dragged items - a  
"check" cursor
The "check" cursor is a new one I added, the other three are the  
standard cursors displayed by the platform.

- Donn