Titlebar widgeting spec finished

[copy of this gitlab issue
<https://gitlab.freedesktop.org/xdg/xdg-specs/-/issues/100>. I probably
should have posted it here from the start]

This is a follow-up for this issue
<https://gitlab.freedesktop.org/xdg/xdg-specs/-/issues/95> from 2 weeks
ago. Since then, I've written the code for the spec and documented it, and
uploaded it here <https://github.com/sammonius/TbWidgets>.

I've never contributed to XDG (or anything) before, so I really need some
help with what to do next. Is there anyone I should contact to ask if this
can become a specification?

EDIT: Should I just send a pull request?