Set enable_blur_behind hint on demand.

Summary:
Creating a window handle is relatively slow. We don't want it in the
constructor if we don't need it.

Replaces D8845

Test Plan:
Opened on X and Wayland. Looked the same as before.
(which to me isn't very blurry, but a regular grey?)

Reviewers: elvisangelaccio, broulik

Reviewed By: elvisangelaccio, broulik

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D8851