[effects/slide] Simplify logic that forces blur and background contrast

Summary:
The "Force" part in WindowForceBackgroundContrastRole and WindowForceBlurRole
is confusing. If WindowForceBlurRole is set on a window, background
behind it won't be blurred.

By setting WindowForceBlurRole we tell the Blur effect "that's fine to
do your business behind transformed windows."

The Slide effect only translates windows, it doesn't distort them, etc.
So, we can set WindowForceBackgroundContrastRole and WindowForceBlurRole
for all windows.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D15707