Add Flatpak manifest

[tinywrkb]

This adds a Flatpak packaging manifest for building the app directly with flatpak-builder from the local folder as the source.

The other difference from the Flathub app is that this builds with the most recent Vulkan headers, and bundles the latest vulkan-tools tagged release. The Flathub app will catch up with this change a bit later.

All the modules' sources in the manifest have x-checker-data properties for easily updating the sources with flatpak-external-data-checker (f-e-d-c). e.g.

flatpak-external-data-checker --edit-only io.github.arunsivaramanneo.GPUViewer.yaml

Note that f-e-d-c is available as the Flatpak app org.flathub.flatpak-external-data-checker.

Some projects, like Gnome Music, package the development builds with a different application ID than the stable Flathub app.
This requires changing filenames (desktop file, icon, metainfo), Name entry in desktop file, etc.
At this stage, I preferred to avoid complicating the Meson files, and keep the same application ID.

The other methods to distinguish between multiple versions of the same Flatpak app are to install them to different Flatpak installation location (system, user, or custom), and using different Flatpak branches.
I usually use the first method, installing Flathub apps into the system Flatpak installation, and anything else into the user Flatpak installation.