Move to Gradle-based project build

[arminzavada]

https://github.com/ftsrg/refinery uses Gradle successfuly to automate the different xtext-xtend and emf code generation, multiple plugins/projects, etc.

Currently, it is a long and hard process for someone to start using the project from scratch. Also, currently it is not possible for us to automate the building process on GitHub side, which would enhance the development process.

We should set up a similar build on this side as well, in order to simplify the development and integration process. In the future, this could be extended with GitHub actions, Sonarqube, JaCoCo, etc.