prepare version 2.0.0 release

[christoff-buerger]

pmChess is ready for the next release, version 2.0.0. To that end:

• A classic chess match shall be used for screenshots. A good candidate is Game Of The Century | Byrne vs Fischer (1956). The adjourned-game.data binary should be saved in the releases/version-2.0.0 directory. An accompanying adjourned-game.txt documenting which game is stored shall be added.
• Screenshots showing normal-mode, history-mode and the About pmChess dialog should be provided for macOS and Windows. They should be saved in the releases/version-2.0.0 directory.
  • The readme has to be updated to use the new screenshots.
• The release notes have to be copied to the releases/version-2.0.0 directory.
• The macOS, Windows and portable jar distributions have to be build, the .gitignore entry preventing their distribution with the pmChess repository has to be removed (changed to releases/version-3.0.0) and the bundles be pushed.
• The v2.0.0 tag has to be added.