Add flatpak as a download option

[hfiguiere]

I submitted LDView for inclusion on Flathub to be installed as a flatpak.

See flathub/flathub#2844

When it is available there, it would be nice to have this option listed for Linux user to install LDView.

Also let me know if you want to be added to the repository so you can update (not required, but we like to give the option to upstream developers).

There are already a couple of other LEGO app on Flathub, like LeoCAD and LPub3D.