Add draft flatpak manifest (#4468)

Packaging/nix/org.diasurgical.DevilutionX.yml

@@ -0,0 +1,26 @@
+app-id: org.diasurgical.DevilutionX
+runtime: org.freedesktop.Platform
+runtime-version: '21.08'
+sdk: org.freedesktop.Sdk
+command: devilutionx
+rename-desktop-file: devilutionx.desktop
+finish-args:
+    - --socket=fallback-x11
+    - --socket=wayland
+    - --device=dri
+    - --share=network
+    - --socket=pulseaudio
+    - --filesystem=xdg-config
+modules:
+    - name: devilutionX
+      buildsystem: cmake-ninja
+      config-opts:
+        - -DBUILD_TESTING=OFF
+        - -DCMAKE_BUILD_TYPE=Release
+      sources:
+        - type: archive
+          path: ./devilutionx-src-full-1.4.0-aab970dc.tar.xz
+      post-install:
+        - "sed -i 's/Icon=devilutionx-hellfire/Icon=org.diasurgical.DevilutionX-hellfire/g' ${FLATPAK_DEST}/share/applications/devilutionx-hellfire.desktop"
+        - "mv ${FLATPAK_DEST}/share/applications/devilutionx-hellfire.desktop ${FLATPAK_DEST}/share/applications/${FLATPAK_ID}-hellfire.desktop"
+        - "mv ${FLATPAK_DEST}/share/icons/hicolor/512x512/apps/devilutionx-hellfire.png ${FLATPAK_DEST}/share/icons/hicolor/512x512/apps/${FLATPAK_ID}-hellfire.png"