Start of improved applet drag & drop. What works now is:
* dragging an applet creates a top level window so the applet won't appear behind the panel when dragging, as is the case now.
* dragging leaves a semi transparent version of the applet at the original location, so it's clear to the user where the applet came from and where it will go when the drag is cancelled.

There are still some open issues, I'm working on those...