1256 Track Browser column header drag'n'drop issues [#5842]

[chrisjj]

EDIT: Each Track Browser column header is draggable, but

1 though other TB column header positions offer drop, they do not accept. I suggest they should accept. Else not offer.
2 invalid positions e.g. in other panes offer (but do not accept) drop. I suggest they should not offer.

[chrisjj]

Tracked at http://www.ventismedia.com/mantis/view.php?id=5842