Add Flatpak CI/CD Integration

Adds flatpak manifest file and integrates flatpak ci with the application. @apol @ravier

.flatpak-manifest.json

@@ -0,0 +1,37 @@
+{
+    "id": "org.kde.falkon",
+    "runtime": "org.kde.Platform",
+    "runtime-version": "5.15-22.08",
+    "base": "io.qt.qtwebengine.BaseApp",
+    "base-version": "5.15-22.08",
+    "sdk": "org.kde.Sdk",
+    "command": "falkon",
+    "rename-icon": "falkon",
+    "finish-args": [
+        "--share=ipc",
+        "--share=network",
+        "--socket=fallback-x11",
+        "--socket=wayland",
+        "--socket=pulseaudio",
+        "--device=dri",
+        "--system-talk-name=org.freedesktop.GeoClue2",
+        "--system-talk-name=org.freedesktop.UPower.*",
+        "--talk-name=org.kde.kwalletd5",
+        "--own-name=org.mpris.MediaPlayer2.falkon.*"
+    ],
+    "modules": [
+        {
+            "buildsystem": "cmake-ninja",
+            "name": "falkon",
+            "config-opts": [
+                "-DENABLE_TESTING=OFF"
+            ],
+            "sources": [
+                {
+                    "type": "dir",
+                    "path": "."
+                }
+            ]
+        }
+    ]
+}

.flatpak-manifest.json.license

@@ -0,0 +1,2 @@
+# SPDX-FileCopyrightText: None
+# SPDX-License-Identifier: CC0-1.0

.gitlab-ci.yml

@@ -1,3 +1,4 @@
 include:
   - https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/linux.yml
   - https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/freebsd.yml
+  - https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/flatpak.yml