Create desktop@ as maintainer of some shared desktop ports

[tcberner]

Summary

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.

Todo: write wiki entry.

[tcberner]

To clarify: the idea is that members of gnome@, kde@ and other desktopy-thingy@ are implicit members of this group.

[swills]

LGTM

[tcberner]

Wiki entry: https://wiki.freebsd.org/DesktopTeam

[adamw]

I applaud this move. It sets a great example of where individual maintainership should head: shared responsibility across the consumers who need to leverage it.