Add GitHub Actions workflow to build and publish Flatpak bundle

Agent-Logs-Url: https://github.com/f4exb/sdrangel/sessions/862cd268-1d4c-402d-b530-b2885bf6d153

Co-authored-by: srcejon <57259258+srcejon@users.noreply.github.com>

.github/workflows/flatpak.yml

@@ -0,0 +1,49 @@
+name: SDRangel Flatpak build
+
+on:
+  push:
+    branches:
+      - flatpak
+    tags:
+      - 'v*'
+  workflow_dispatch:
+
+jobs:
+  build_flatpak:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          submodules: true
+          fetch-depth: 0
+      - name: Get version
+        id: get_version
+        run: |
+          if [[ "${{ github.ref_name }}" == "flatpak" ]]; then
+            echo "version=${{ github.sha }}" >> "$GITHUB_OUTPUT"
+          else
+            echo "version=$(echo "${{ github.ref_name }}" | cut -c2-)" >> "$GITHUB_OUTPUT"
+          fi
+      - name: Install dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y flatpak flatpak-builder
+      - name: Add Flathub remote
+        run: |
+          flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
+      - name: Build Flatpak
+        uses: flatpak/flatpak-github-actions/flatpak-builder@v6
+        with:
+          bundle: sdrangel-${{ steps.get_version.outputs.version }}-x86_64.flatpak
+          manifest-path: flatpak/org.sdrangel.SDRangel.yaml
+          cache-key: flatpak-builder-${{ github.sha }}
+      - name: Upload artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: sdrangel-${{ steps.get_version.outputs.version }}-x86_64.flatpak
+          path: sdrangel-${{ steps.get_version.outputs.version }}-x86_64.flatpak
+      - name: Upload release
+        if: startsWith(github.ref, 'refs/tags/')
+        uses: softprops/action-gh-release@v2
+        with:
+          files: sdrangel-${{ steps.get_version.outputs.version }}-x86_64.flatpak