[effects/morphingpopups] Use enumerators from the DataRole enum

Summary:
Use Effect.WindowForceBackgroundContrastRole and Effect.WindowForceBlurRole
instead of integers to improve readability.

Reviewers: #kwin, broulik

Reviewed By: broulik

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15709