[REVIEW] Improved applet drag&drop support

[Rob Scheepmaker]

Hi,

I've improved my drag&drop patch some more. This patch tries to improve 
the drag and drop experience by showing the applet on a top level 
window so it won't disappear behind the panel when dragging, as is the 
case now. The original applet is made semi transparent to leave a 
ghosted image where the applet came from. This way it's clear to the 
user what happens when the drag is cancelled.

This feature will still need some improvement, but this patch seems to
handle the basics quite well so I want to commit it.

There's a couple of stuff that I'll improve later:
* when zoomed out, the toplevel window gets the correct size, but it
  shows only the bottom right corner of the applet at its original
  size.
* we probably only want to move to a toplevel window when necesarry
  (with transparency in that case being ugly for non compositing users
  and what not)
* Speaking of transparency: the background of the applet are those
  nice "this is transparent" tiles. Those are being drawn by corona, so
  a fix is needed there.
* add a previewAddApplet function or something like that to containment,
  so that can be called while dragging over a panel. The panel can
  implement this function to add a spacer on the location in the layout
  where the applet lands when being dropped.

There's probably some other stuff to fix too, but I wanted to at least
commit this. Feedback is very welcome! :)

Cheers,

Rob Scheepmaker
(pinda)