[feature request] draggable layers in treePanel

[albell]

Frank shared some code to address this on the mailing list:

http://www.geoext.org/pipermail/users/2013-February/003225.html

but it appears to reinvent the wheel, and I haven't figured out how to adapt it yet for my project. This is such a basic feature requirement that it ideally it would be supported out-of-the-box, so I thought I'd inquire.

While I'm making up my christmas list, let me also wish for some config options to tame some of the complexities involved:

allowDragOfParent: boolean // default false?

Useful for keeping the "base" layer group always below below an "overlay" group, for example.

allowDragOntoNewParent: boolean // default false?

This would prevent, for example, a "base" layer from being dragged into an "overlay" layer, messing up the default nesting.

onDragError: function()

A callback called when the user does a disallowed drag.