21.3.0: Feature-Ergänzung & viele Fixes

Implementierung fehlender Funktionen (klonen, TestClient, toString) und viele Fixes

Fixed

• Send final Gamestate to listeners when a game ends (#364)
• Remove extra field from Move/Piece XML (23589a153)
• Player dependency declarations (#373)

Added

• Implement cloning for GameState & Board (#356)
• Add README for player (#373)
• Create CONTRIBUTING & GUIDELINES (#360)
• Modularize XStream initialization using ServiceLoader (#352)

Changed

• Improve logback config (#371)
• Improve Gradle configuration & TestClient build (#368)
• TestClient: Make it work (#372) & load current Game id from classpath (#367)