Install symbolic app icon

This makes the icon available to notification; GNOME now automatically uses the
symbolic icon.

Closes #69

flatpak/de.swsnr.turnon.Devel.yaml

@@ -35,6 +35,7 @@ modules:
       # Install binary, app icon, desktop file, appstream metadata, DBus service, and search provider
       - install -Dm0755 target/release/turnon /app/bin/de.swsnr.turnon.Devel
       - install -Dm0644 -t /app/share/icons/hicolor/scalable/apps/ resources/icons/scalable/apps/de.swsnr.turnon.Devel.svg
+      - install -Dm0644 resources/icons/symbolic/apps/de.swsnr.turnon-symbolic.svg /app/share/icons/hicolor/symbolic/apps/de.swsnr.turnon.Devel-symbolic.svg
       - install -Dm0644 de.swsnr.turnon.desktop /app/share/applications/de.swsnr.turnon.Devel.desktop
       - install -Dm0644 resources/de.swsnr.turnon.metainfo.xml  /app/share/metainfo/de.swsnr.turnon.Devel.metainfo.xml
       - install -Dm0644 dbus-1/de.swsnr.turnon.service /app/share/dbus-1/services/de.swsnr.turnon.Devel.service