D29815: Fix blurry icons in titlebar appmenu by adding UseHighDpiPixmaps flag

[Matej Mrenica]

mthw added a reviewer: Frameworks.

REPOSITORY
  R297 KDED

REVISION DETAIL
  https://phabricator.kde.org/D29815

To: mthw, #frameworks
Cc: kde-frameworks-devel, LeGast00n, cblack, michaelh, ngraham, bruns