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, add target platform

Creates the basic language infrastructure from the Xtext grammar
and compiles Xtend sources to Java files. The target platform can
be used to switch Eclipse to the supported environment with one
click ('Set as Target Platform' in the Target Editor).
Merge pull request #1 from fsteeg/build

Clean build with Tycho