Storing JGraphs and the new project model

[Andrus Adamchik]

Looks like we'll need to settle on a more permanent solution for  
storing the graph XML file sooner than I expected. Currently there's  
work being done on switching the Modeler to the new 3.1 project  
structure. This task is a blocker for many other 3.1 improvements, so  
we need to finish the switch ASAP.

The switch will introduce the new configuration model with one  
DataDomain per configuration stack. This means that the current  
mechanism for storing JGraph XML file, as implemented in  
org.apache.cayenne.modeler.ModelerProject will no longer work. We can  
probably make it work with the new API using some other means, but I'd  
rather we have a clear picture first.

I still prefer the solution that I suggested before - store the layout  
files under ~/.cayenne/, using generated unique names tied via  
preferences to the filesystem location of a given project. Later we  
can expand that to allow "Save As / Open" features, etc. to allow  
users more control, but for now this should be the least invasive  
option that would allow us to evolve the graph engine in the  
background without affecting the users' ability to work with Modeler  
3.1.

Thoughts?
Andrus