Refactor yangster to use the eclipse versions of sprotty and sprotty-theia

As suggested [here](https://github.com/theia-ide/yangster/issues/69#issuecomment-444490848), now that Sprotty has [moved](https://github.com/eclipse/sprotty) under the Eclipse Foundation, Yansgter should make use of that version of sprotty and sprotty-theia (the original [repo](https://github.com/theia-ide/sprotty) is now read-only, so there will be no more releases from there). 