add a DSL registry based on dsl files

in order to fully enable #8 and #57 we need a registry in eclipse that can reference the languages declared in dsl files.

Ie. similarly to Melange registry, it needs an extension point that plugin providing a dsl file must use in order to expose the language.

It will only declare the dsl, without any information about dsl relation ship (extends/implements, ...)
These relation ship would be computed only in the running eclipse at request amoung available installed languages