modified the Descriptor persistence scheme.

We used to use Map<String,Object> as a data store for Descriptor, but this requires a lot of boiler plate code to move values in/out from this Map, making the code uglier.
I modified this to just use XStream like the rest of the Hudson, so that the information can be persisted by simply storing them in fields.
Existing Descriptors are enhanced so that they can convert formats from pre-1.62 XML format.


git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@1126 71c3de6d-444a-0410-be80-ed276b4c234a