rebase of desktop proguard

I've rebased last pull request about proguard for desktop. There were few minor mistakes, not I've actually run more tests (including applications with assets). It cuts simple project by 10mb (to 6 from 16), most of which is platform dependent stuff.

I think that separate building of platform dependent libraries can be considered later.

(note: This was https://github.com/ajhager/libgdx-sbt-project.g8/pull/50 but I decided that mavenization of project touched build.scala in so many places,that it was easier to make new, fresh pull request. I'm linking it, so discussion below it does not disappear).