Metatext: Xtext-based editor and Eclipse integration for Metaflow

Initial implementation: usable, much potential for improvements.

- Xtext-based editor for the Culturegraph Metaflow language
- Eclipse launcher to run *.flow files directly (Run As -> Metatext)
- Tycho-based Maven build with p2 repo and product generation

See basic usage documentation in metatext/README.textile.