Fundamentals: more specific use of the desktop

Summary:
Drop Workspaces usage; prefer &plasma-desktop; to &plasma-workspaces;.
Refer to the KDE software in a more generic way (applications and the
desktop).

Reviewers: #documentation, lueck

Reviewed By: #documentation, lueck

Subscribers: lueck, #documentation

Differential Revision: https://phabricator.kde.org/D1959