build: add AppImage packaging script

packaging/appimage/AppRun

@@ -0,0 +1,11 @@
+#!/bin/sh
+# Start the real binary by its path inside the AppImage. A symlinked AppRun makes Qt take
+# the AppImage root as the application dir, so it misses usr/bin/qt.conf and finds no
+# platform plugins.
+HERE="$(dirname "$(readlink -f "$0")")"
+# The Plasma platform theme's own file dialog lists folders through KIO workers, which are
+# not bundled (it would show every folder as empty). Use the host's dialog via the
+# xdg-desktop-portal instead.
+export PLASMA_INTEGRATION_USE_PORTAL=1
+export XDG_DATA_DIRS="$HERE/usr/share:${XDG_DATA_DIRS:-/usr/local/share:/usr/share}"
+exec "$HERE/usr/bin/qtpotato" "$@"