Add versioning (WIP)

* Gradle Git Versioning Plugin
* Changelog
* Flatpack build workflow

.gitea/workflows/flatpak.yaml

@@ -0,0 +1,34 @@
+on:
+  push:
+    # TODO: remove versioning branch once everything works
+    branches: [main,versioning]
+    tags:
+      - '[0-9]+.[0-9]+.[0-9]+'
+name: Flatpak
+jobs:
+  flatpak:
+    name: Flatpak
+    runs-on: ubuntu-latest
+    container:
+      image: ghcr.io/flathub-infra/flatpak-github-actions:gnome-48
+      options: --privileged
+    steps:
+      - uses: actions/checkout@v4
+      - name: Set up JDK 21
+        uses: actions/setup-java@v4
+        with:
+          java-version: 21
+          distribution: 'temurin'
+      - name: Cache Gradle packages
+        uses: actions/cache@v4
+        with:
+          path: ~/.gradle/caches
+          key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle') }}
+          restore-keys: ${{ runner.os }}-gradle
+      - name: Build Release Uber JAR
+        run: ./gradlew packageReleaseUberJarForCurrentOS
+      - uses: flatpak/flatpak-github-actions/flatpak-builder@v6
+        with:
+          bundle: YAEP.flatpak
+          manifest-path: ./flatpak/ch.dissem.YAEP.yml
+          cache-key: flatpak-builder-${{ github.sha }}