D11026: [taskmanager applet] Mark some caching QML Item properties readonly

[Kai Uwe Broulik]

broulik added a reviewer: hein.

REPOSITORY
  R119 Plasma Desktop

REVISION DETAIL
  https://phabricator.kde.org/D11026

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