Allow items to be moved (not copied) between collections w/modifier key

Cmd on OS X, Shift on Windows/Linux

How do I not get to close a ticket for this?

Unfortunately on Windows it doesn't seem possible to set the cursor
effect to arbitrary states (see note in libraryTreeView.js::
_setDropEffect() for the gory details), so this just uses the default
cursor there. On OS X and Linux the cursor reflects the requested
action.