build.scala cleanup

After some work I found out, that for tests, gdx isn't needed at all. I believe
tests works by calling other application (that is why app must be installed
before installing/running tests), so it needs only very thin layout. So, this
commit just cleans up build.scala a bit and adds proguard.cfg with Scala
related stuff to tests. This file will be mostly static for whole project life,
so it is currenlty best solution. I checked tests building (no warnings at all)
and tests itself (all tests passed).