Use enthought's envisage as the foundation for xpaxs

Enthought has been developing a framework for scientific applications, based on traits. Their envisage package can be used to create applications with plugins, and it supports a qt4 backend. Dock windows are supported. There are a few simple example applications we can use to guide our own development. Enthoughts code is BSD, and this framework actually appears to be very well suited for the work we are trying to do. Their event model is a little different than what we use, but traits event model has lots of enthusiastic support. Since the qt support was largely done by the author of PyQt4, it should be pretty robust. One issue is the completeness of the qt4 traits backend, enthought's focus has been on wx. This emphasis has changed recently, especially now that Qt and PySide are available under the conditions of the LGPL.