D8499: Constify some methods

[Laurent Montel]

mlaurent added a reviewer: hein.

REPOSITORY
  R119 Plasma Desktop

REVISION DETAIL
  https://phabricator.kde.org/D8499

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