Update to Xtext 2.13 and cleanup @szarnekow's changes

[mmews-n4]

When fixing #39, @szarnekow introduced changes that can be reverted after updating to XText 2.13. To do this, search for Xtext 2.13 / 2.13 or refer to pull request's changes.

[yoosiba]

should be tackled after #637 (#370) is merged.

[yoosiba]

Note in many places in code there is TODO remove with Xtext 2.13 that should be searched for when working on this task. See:
https://github.com/eclipse/n4js/pull/138/files#diff-9123c7830f32f5a7edf66df8e7a7f0d5R25
https://github.com/eclipse/n4js/pull/65/files#diff-98014d08661739d97fbc0c75eb0c7f64R358

[szarnekow]

All the changes marked with expected to be part of the framework in 2.13 didn't make it into 2.13. Everything was part of 2.14 though.