Add drag-n-drop support to be able to re-integrate the dive list

Add drag-n-drop support to be able to re-integrate the dive list

This is somewhat hacky, and there is clearly something I still don't
understand about gtk selections and drag-n-drop.  Dropping it back
works, but I get a nasty error when I do it:

   (subsurface:8512): Gtk-CRITICAL **: IA__gtk_selection_data_set: assertion `length <= 0' failed

even though I actually never set any selection at all directly.  So
there must be some internal gtk rule that I am violating, but I can't
see what it is.

I probably shouldn't commit it with a known ugly wart like that, but I
really have no clue.  Maybe somebody else can figure out what is up.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>