Publish Quba to Flathub

[ClFeSc]

We'd like to use Quba in our organization. To allow for easier deployment it'd be nice to publish Quba to Flathub.

We have supplied a Manifest file for building Quba in a flatpak, see flatpak/org.quba_viewer.Quba.yml. The chosen app id org.quba_viewer.Quba is completely up for debate, if you think another name would be better.

Instructions to build it locally have been added to the README:

# Build & Install
cd flatpak
flatpak-builder build org.quba_viewer.Quba.yml --install-deps-from=flathub --force-clean --user --install

# Run
flatpak run org.quba_viewer.Quba

In case you're interested in publishing Quba to Flathub, we'd be willing to support you in the process.