Clarify terminology: KDE is the community, Plasma is the desktop

- use the correct name (Plasma) for the desktop environment;
- soften the dependency of applications by KDE on the desktop (there
  is none);
- highlights that KDE is the "producer" when required.

[hertzog@debian.org: Minor tweaks to merge with former independant
changes and to explain IDE before using the term]

Closes: #860477
Signed-off-by: Raphaël Hertzog <hertzog@debian.org>