Patch for reparenting accounts through drag and drop

[Matt Lavin]

One of the missing features I first hit when trying out Gnucash was the
lack of drag and drop for changing an account's parent.  After importing
from Quicken I had a lot of accounts I wanted re-parented, and drag and
drop would have been nice.  

I thought that trying to add that feature would be a nice contained bit
of work so I gave it a shot and got it working.  I've attached the patch
to  https://bugzilla.gnome.org/show_bug.cgi?id=121695 .  I'd love to get
any feedback on things I could improve so that the drag and drop support
could be committed to SVN.