Add flatpak testing workflow

Builds nightly with latest GStreamer from git (no patches applied)

.github/workflows/flatpak-testing.yml

@@ -0,0 +1,21 @@
+on:
+  schedule:
+    - cron: "0 0 * * *"
+name: "Flatpak Testing"
+jobs:
+  flatpak:
+    name: "Flatpak"
+    runs-on: ubuntu-latest
+    container:
+      image: bilelmoussaoui/flatpak-github-actions:gnome-40
+      options: --privileged
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        submodules: recursive
+    - uses: bilelmoussaoui/flatpak-github-actions/flatpak-builder@v3
+      name: "Build"
+      with:
+        bundle: com.github.rafostar.Clapper.flatpak
+        manifest-path: pkgs/flatpak/com.github.rafostar.Clapper-testing.json
+        cache-key: flatpak-builder-${{ github.sha }}