menu-spec: add DDE to Registered OnlyShowIn Environments list

This patch adds DDE to registered OnlyShowIn environments list.

I also started a discussion in deepin's developer mailing list about this topic, and I'll remove the draft marker when the discussion is finished and we're ready to merge (edit: it's ready now).

Related:

• !68 (merged)
• https://github.com/PackageKit/PackageKit/pull/603
• https://www.freelists.org/post/deepin-devel/xdgspec-DDE (Chinese)
• https://github.com/linuxdeepin/developer-center/issues/3829 (Chinese)