Clean build with Tycho

**Delete generated files and add their locations to `.gitignore`**

All generated resources should be created as part of a clean build and not be part of the versioned source files, eg. for clean diffs.

**Generate Xtext and Xtend sources during build**

Create the basic language infrastructure from the Xtext grammar and compile Xtend sources to Java files.

**Add target platform**

The target platform can be used to switch Eclipse to the supported environment with one click ('Set as Target Platform' in the Target Editor).