[Linux] PoC Flatpak packaging

[tinywrkb]

I saw the latest FOSSASIA video update about the app the other day and thought I'll try and package the app as it might be something that I'm gonna use in the future.
This is pretty much untested as I don't have the HW.

Even if this is working correctly, this is definitely not ready for publishing on Flathub.org. There are a couple of issues and todos that are needed to be taken care of first:

• I had to regenerate package-lock.json as the build failed with ELIFECYCLE error.
• Offline build is broken, I haven't figure out why. I needed to enable networking during the build, which is against Flathub's guidelines.
  This is not a blocker for submitting the app to Flathub as it's possible to re-use a binary release instead of building from source, and just repackage the app, but it would be better to fix this as Flathub build servers also building for aarch64.
• Appstream metadata is missing and should be added.
• I'm not sure if all the needed depedendencies were packaged. Maybe replacing blas with openblas.
• Other minor issues like upstreaming the desktop file, app icon in more sizes. I believe at least 128x128 is needed for Flathub submission.

Flatpak manifest

You gonna need to build the Flatpak manifest with flatpak-builder.
The sources manifests, generated-sources.json and python-dependencies.json were generated with flatpak-builder-tools

edit: I updated the Flatpak manifest link.

[tinywrkb]

It should be noted that Electron Builder recently gained support for building Flatpak bundles directly but won't fit for distributing via Flathub.