Add Flatpak manifest

com.collabora.gtimelog.json

+{
+  "app-id": "com.collabora.gtimelog",
+  "runtime": "org.gnome.Platform",
+  "runtime-version": "3.36",
+  "sdk": "org.gnome.Sdk",
+  "command": "gtimelog",
+  "finish-args": [
+    "--socket=fallback-x11",
+    "--socket=wayland",
+    "--share=ipc",
+    "--filesystem=~/.gtimelog/:ro",
+    "--share=network"
+  ],
+  "modules": [
+    {
+      "name": "gtimelog",
+      "buildsystem": "simple",
+      "build-commands": [
+        "python3 setup.py install --prefix=/app --root=/",
+        "install -D src/gtimelog/gtimelog.png /app/share/icons/hicolor/48x48/apps/com.collabora.gtimelog.png",
+        "install -D gtimelog.desktop /app/share/applications/com.collabora.gtimelog.desktop"
+      ],
+      "sources": [
+          {
+          "type": "git",
+          "url": "https://gitlab.collabora.com/tintou/gtimelog.git"
+          }
+      ]
+    }
+  ]
+}