New features and fixes

• Don't use Quaqua anymore, which should fix some crashes on macOS (issue #18).
• Support for large boards up to 52x52.
• Support for games with moving pieces such as chess or ataxx
• Drag-and-drop of sgf files (thanks to @zakki)
• Fix wrong player names in sgf from gogui-twogtp -alternate (issue #9, thanks to @tomtseng)
• New gogui-rules_captured_count command.
• Minor fixes and improvements.