JavaSourceCategory is no longer public

In GWT 7.0 com.google.gwt.user.rebind.ClassSourceFileComposerFactory.JavaSourceCategory seems to be no longer public, so I added the Enum to resty-gwt in order to use it with GWT 7.0-rc1.

https://github.com/gwtproject/gwt/commit/e2154f39479e29dfd50ea27dc79ddaec782bdb90