Add versioning (WIP)

* Gradle Git Versioning Plugin
* Changelog
* Flatpack build workflow

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+
+- Yet Another Einstein Puzzle, for Desktop and not-quite-yet Android
+
+### Changed
+
+### Deprecated
+
+### Removed
+
+### Fixed
+
+### Security
+
+
+[unreleased]: https://git.dissem.ch/chris/YAEP/compare/1b9c8633c96e479e63afcd7301125ce67247a198...HEAD
+[0.0.0]: https://git.dissem.ch/chris/YAEP/releases/tag/0.0.0