Improve default proguard setup

This set of commits improves default proguard setup. There is summary of changes:

- allow comments in proguard file
- eliminate all warnings for empty project, i.e.
  - add descriptor classes needed for LibGDX
  - add field kept by means of introspection
- add workaround for hard to find but easy to trigger bug https://issues.scala-lang.org/browse/SI-5397 (I was hitting it with every non trivial code, like using Future[...] to load text file).