Add treeselector widget

[flack]

Reported by flack on 13 Jan 2011 13:58 UTC ATM, chooser is used in many forms for selecting linked items from a hierarchical structure. This works in some cases, but in even more cases, it would be much simpler to have a filesystem explorer-like widget that can browse thorught structures (e.g. projects and tasks). Something like this can probably be built on top of dynatree, but maybe aa more readymade solution can be found.

Migrated-From: http://trac.openpsa2.org/ticket/156