[gpp] beyond the basic support

collecting together a few tasks that could make life easier.

be good to do:

1) Nature and actions for add/remove groovy++ nature on a project.
2) As well as managing nature, manage add/remove of groovypp jar from the groovy classpath container (we include a compatible version of groovy++ in the plugin)
3) Allow the user to override that level with their own groovypp jar (at least tolerate it) - preference page for pointing at a diff version?
4) Create content type for gpp source in a groovy project - wire .gpp/.grunit to it, they are currently wired to the one existing groovy content type
5) menu actions for quickly moving files to/from .groovy/.gpp

might do:

6) New gpp/grunit file wizard? (or just an option on the existing wizard?)

gpp is far more important to me than grunit, so that would get priority for all these things.