Add ability to build GTK app AppImage (#240)

#238 Add AppImage build support

Implements

- Downgrade to libadwaita 1.3 for wider distro support
- Add build script, workflow, and docs for AppImage
- Add build docs for Debian (apt) and Void Linux
- Building AppImage in CI

.github/workflows/build-appimage.yml

@@ -0,0 +1,23 @@
+name: Build AppImage
+on:
+  push:
+    branches: [main]
+  pull_request:
+jobs:
+  appimage:
+    name: Build AppImage
+    runs-on: ubuntu-latest
+    container: docker
+    steps:
+    - uses: actions/checkout@v4
+    - name: Build AppImage
+      run: |
+        docker build -t appimage-builder . -f burrow-gtk/build-aux/Dockerfile
+        docker create --name temp appimage-builder
+        docker cp temp:/app/burrow-gtk/build-appimage/Burrow-x86_64.AppImage .
+        docker rm temp
+    - uses: actions/upload-artifact@v4
+      with:
+        name: AppImage
+        path: Burrow-x86_64.AppImage
+