https://gitlab.xfce.org/xfce/libxfce4ui/-/tags/libxfce4ui-4.20.1

Should already be the case by default, but better be explicit

Context: https://archlinux.org/todo/general-xfce-packages-rework/

- Update URL (point to upstream doc webpage for the package)
- Use SPDX identifier for the license
- Switch to a more transparent source (git sources)
- Add check()
- Add nvchecker integration (.nvchecker.toml)

Ref: https://mail.xfce.org/pipermail/xfce4-dev/2017-December/032085.html

Note: https://archive.xfce.org/ might still redirect to http:// mirrors
      but that should be fixed in the near future to redirect to https://