D20261: Fix Plasma themes' quassel icon to match current quassel icon names

[Aleix Pol Gonzalez]

apol added a reviewer: VDG.

REPOSITORY
  R242 Plasma Framework (Library)

REVISION DETAIL
  https://phabricator.kde.org/D20261

To: kossebau, #plasma, #vdg
Cc: kde-frameworks-devel, michaelh, ngraham, bruns