Add Theme::blurBehindEnabled()

Summary:
Allows themes to disable the blurring of what is behind panels,
if they either do not need it (being fully opaque) or by design want
non-blurry transparency.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D19907