Create desktop@ as maintainer of some shared desktop ports

This idea has been around for quite some time. Time to make it happen.

In order to share the load on the ports required by multiple desktop
environments start to share the responsibility of maintainership.

This is the initial list that came to mind, but we can probably extend it, to
include another handful of ports.

WWW: https://wiki.freebsd.org/DesktopTeam
Mailing List: https://lists.freebsd.org/mailman/listinfo/freebsd-desktop

Approved by: swills, kwm (gnome), madpilot (xfce)
Differential Revision: https://reviews.freebsd.org/D22389