Integrate Xturtle, a Turtle editor from AKSW Leipzig (see #6)

This makes the Turtle editor available in the Metafacture IDE (for *.ttl files). The editor is consumed from a p2 zip file on our server, which is built with Tycho using a fork of the Xturtle repo (I've opened a pull request for adding the Tycho build: https://github.com/AKSW/Xturtle/pull/53).