build: Add a Flatpak manifest

This allows building the master branch of QElectroTech with a simple
command:

$ flatpak-builder _build build-aux/flatpak/org.qelectrotech.QElectroTech.json

build-aux/flatpak/org.qelectrotech.QElectroTech.json

@@ -0,0 +1,37 @@
+{
+  "id": "org.qelectrotech.QElectroTech",
+  "runtime": "org.kde.Platform",
+  "runtime-version": "5.14",
+  "sdk": "org.kde.Sdk",
+  "command": "qelectrotech",
+  "rename-desktop-file": "qelectrotech.desktop",
+  "rename-appdata-file": "qelectrotech",
+  "rename-icon": "qelectrotech",
+  "copy-icon": true,
+  "finish-args": [
+    "--socket=wayland",
+    "--socket=x11",
+    "--device=dri",
+    "--share=ipc",
+    "--filesystem=host"
+  ],
+  "modules": [
+    {
+      "name": "qelectrotech",
+      "buildsystem": "qmake",
+      "post-install": [
+        "mv /app/share/mime/packages/qelectrotech.xml /app/share/mime/packages/org.qelectrotech.QElectroTech.xml"
+      ],
+      "sources": [
+        {
+          "type": "dir",
+          "path": "../.."
+        },
+        {
+          "type": "patch",
+          "path": "patches/0001-build-Fix-the-installation-paths.patch"
+        }
+      ]
+    }
+  ]
+}