Enable blurBehind effect if the window hints for it

The new `ENABLE_BLUR_BEHIND_HINT` property can be used to enable the
blur-behind effect in widgets that cannot use KWindowEffects (for
example, widgets in tier1 frameworks). The variable is honored in both
the x11 and wayland platform themes.

It is currently set by KToolTipWidget in kwidgetsaddons.

Reviewers: graesslin

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3714