[effects] replace old slide effect with a new one

Summary:
The new slide effect tries to separate each virtual desktop
as much as possible. This separation makes the new slide
effect more intuitive than the old one.

Test Plan:

• switch between virtual desktops
• or, move a window to another virtual desktop

Reviewers: VDG, KWin, Plasma, graesslin, ngraham

Reviewed By: KWin, Plasma, graesslin

Subscribers: mart, graesslin, abetts, ngraham, plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D9638