dnd.js: remove some dead code
dnd: Fix drag and drop getting stuck when moving out of the confines of a panel
or a menu.  The drag actor is now visible wherever you take it, and mouse
ungrabs are registered anywhere.

Also fixed desklet sometimes being selected thru an application window, and
add a mandatory signal-handler for desklets to popModal (some desklets I tested
override onDragEnd, and popModal *must* be run at the end of a drag.)

Fixes #3280
Fixes #2370
Fixes #2224 (final comment)
Fixes #3109

Maybe more?