First Flatpak Manifest for Client Application

With some modifications because of the “special needs“ of ROOT, the
client of Fp311Online can now be built as a Flatpak app.

flatpak/fp311online-client.flatpak.yml

+app-id: de.ruhr_uni_bochum.ep1.tobias.fp311online.client
+runtime: org.kde.Platform
+runtime-version: '5.15'
+sdk: org.kde.Sdk
+command: fp311online-client
+finish-args:
+  # X11 + XShm access
+  - --share=ipc
+  - --socket=fallback-x11
+  # Wayland access
+  - --socket=wayland
+  # Needs to talk to the network:
+  - --share=network
+  # Accelerated graphics access
+  - --device=dri
+modules:
+  - name: fp311online-client
+    buildsystem: cmake
+    config-opts:
+      - -DBUILD_FOR_FLATPAK=YES
+      - -DCMAKE_CXX_STANDARD=17
+      - -DUSE_HOST_ROOT_INSTALL=NO
+    sources:
+      - type: git
+        url: https://gitlab.ep1.rub.de/fp311online/fp311online-client.git
+        branch: master
+    post-install:
+      # Cling will complain if standard library includes are not available...
+      - cp -R /usr/include/* /app/include