[Chameleon] widget for adding gml layer

[He, Matt Yubin]

I am trying to create a widget that can add a layer by uploading a GML
file. But I still don't quite understand how a widget works. 

My plan is to create a writable map session first, then write a layer
description into map file. 

But I have no clue how the map file is managed in a map session, and how
to change it. 

Could someone give me some hint on this? 


Thanks.


Matt