0.6 release

Brain: RandomBrain introduced.
PR: Defined the game name: Leokom Chess.
CI: migrated to Xenial Travis environment.
CI: migrated to OpenJDK.
CI: SonarCloud analyzing is based on JDK11.
Maintainability: split chess-independent notions from chess-dependent (with ArchUnit).
Maintainability: dropped legacy CI support.
Maintainability: local refactorings.

                    The release is dedicated to the Latkes Ovnt (the 5'th day of Hanukkah 5781).