XML for augmenting NetTango models

[NetTango]

This is a big one.

We need to be able to add/remove/show/hide/change things by adding xml files to the models.

We need to make some big decisions on what to include, how to implement it, etc. Amartya and I will meet to talk about it before our meeting on Aug 27th if possible.

[NetTango]

Mike suggested a cascading model that would take into account several xml files for each model - this would make it much easier to switch between different settings (e.g. basic/advanced) or for testing various UI layouts.