D10400: Use KDEInstallDir variables instead of custom calculated install paths

[Aleix Pol Gonzalez]

apol accepted this revision.
This revision is now accepted and ready to land.

REPOSITORY
  R120 Plasma Workspace

BRANCH
  lesshardcodedinstallpaths

REVISION DETAIL
  https://phabricator.kde.org/D10400

To: kossebau, #plasma, apol
Cc: plasma-devel, ZrenBot, progwolff, lesliezhai, ali-mohamed, jensreuterberg, 
abetts, sebas, apol, mart