ci: Build a release tarball (Fixes #312)

On a pull request: build the tarball and add a comment to the PR with a
link to the resource.
On push to master: build the tarball and keep the archive accessible in
the associated GitHub Action.

.github/workflows/release.yml

@@ -0,0 +1,32 @@
+name: "Build release tarball"
+
+on: [pull_request, push]
+
+jobs:
+  build:
+    name: Build release tarball
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check out the repo
+        uses: actions/checkout@v2
+      - name: Build png theme
+        shell: bash
+        run: |
+          sudo apt install -y libgtk-3-dev optipng --no-install-recommends
+          ./configure
+          make
+          cd build
+          tar -cf elementary-xfce.tar.gz *
+      - name: Pull request artifacts
+        if: ${{ github.event_name == 'pull_request' }}
+        uses: gavv/[email protected]
+        with:
+          commit: ${{ github.event.pull_request.head.sha }}
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          artifacts: build/elementary-xfce.tar.gz
+      - name: Upload theme artifact
+        if: ${{ github.event_name == 'push' }}
+        uses: actions/upload-artifact@v3
+          name: elementary-xfce
+          path: build/elementary-xfce.tar.gz