added org.slf4j.api due to
https://bugs.eclipse.org/bugs/show_bug.cgi?id=370411 (but I'm still not
getting log on refex mwe :(
minor clean-up & convenience (new workspace)
fileExtensions="mydsl" in std. builder.participant as per re-generation
(just for consistency)
MultiGeneratorXtextBuilderParticipantTest refactorings