Flatpak package

[hadess]

I recently received a Niimbot B1 printer, and wanted a UI application to use it in addition to the smartphone app, so I started packaging it up for inclusion in Flathub.

The pull request for this work is here:
flathub/flathub#5701

It includes v0.0.47 with the patches from #14 to support my printer

Let me know if you're interested in helping maintain this app there, or have any concerns.