Draft: flatpak: Drop NoDisplay from demo desktop file

Since the demo is being distributed as a Flatpak, it is reasonable to expect the application to be show on the desktop environment.

This is patched in the manifest so the app is not shown when installed as a system library and only appears when built as a Flatpak or installed from the GNOME nigthly repository.

As a newcomer I can't count the times I have had to look at the GTK 4 demo, or Handy Demo (see libhandy!669 (closed)) for ideas, and to familiarize myself with the widgets. Having these two demos on equal footing, as regular Flatpak apps, could further empower developers and drive them towards the latest developments in GTK.