Hi guix!

This patch adds kristall, a qt browser for gemini and gopher.

There were some problems with the latest tagged version (V0.3), such as
directories that had to be created manually before install.  I preferred
to use the latest commit (as the author still does not know when a new
release will happen).  Let me know if it's fine.

Without the wrap-qt-program icons were missing.

guix lint reports no warnings.

Thanks!
Nicolò