Registering xt in the Resource Service Registry

The "xt" DSL does not have an editor (anymore), so there is no corresponding entry in the plugin.xml
that would make the Standalone Registry create an entry for "xt" in the Resource Service Registry.
Therefore, an entry is created for "xt" pointing to the same instance as "xpect".
Registering a custom resource service provider for xt

…doing an injector lookup by language, considering additional modules by using SetupContext
Merge pull request #9 from sjka/xtRSP

Registering xt in the Resource Service Registry