Support for hierarchical option lists

[mpo]

We already have option-lists and dependent-select boxes, but this approach is a bit cumbersome:

• It requires the selected option to be communicated in as many member fields as there are levels.
• Also it prohibits a clear hierarchical listing of the options in one round-trip.

In stead we should think about having one single control that allows

• the end-user to select the multiple levels, down to her option of preference (this might have different visual incarnations: dependent selection lists, a big expandable-colabsable hierarchy view, ...)

• transfers the selected value, either as a single unique-item-id, or else as some hierarchy of selected values (ie. a string-path-like notation, or an array of values)

• and can be fed with the complete list of hierarchical options (although for large option hierachies with deep nesting it will be useful to have some mechanism of lazy-loading sublevels)

This came up as a side thought of the discussion at: ​http://groups.google.com/group/kauri-discuss/browse_thread/thread/d367219a53a52522