Bug 423502 & 428712 - Xtext 2.5 update / Use Xtend Maven Plugin to compile Xtend files

This is a composite pull request containing Bug 423502 & 428712.

Bug 423502: There are some tests missing, but I would like to solve Bug 428193 first.

Bug 428712: The Maven Build is designed due to the approach Sven Efftinge published: https://github.com/svenefftinge/maven-xtext-example/