Fixes #1

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 nvchecker integration (.nvchecker.toml)

devtools 20230105-1 enables the debug option by default.

A follow-up commit will remove debug from alongside other options.

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://