Add option for dumping py_cui interface as xml (machine readable) file

[jwlodek]

Is your feature request related to a problem? Please describe.
Working on a utility for designing py_cui based interfaces, and it uses an xml based format to store information about the interface layout. This is loaded/unloaded on the fly. Ideally there should be a by-directional way to convert from an active interface to a UI xml file and back to python code for the interface. (think QT)

Describe the solution you'd like
There should be a way to dump an existing py_cui interface as a machine readable file, most likely as a function.

Additional context
Add any other context or screenshots about the feature request here.