Add wxFormBuilder to Flathub

[lebao3105]

I can see that in the Releases section there are always a .flatpak file - which is in fact a Flatpak package. I was searching around Flathub but found no wxFormBuilder there.

Isn't allowing users to install using package managers like Flatpak good?

[sodevel]

The packages attached to releases are provided as convenience and are produced automatically by the GitHub CI. Of course it would be nice to have wxFB available on whatever package manager there is, but there are just too many and i simply don't have the ressources to support them.