[effects] Add Scale effect

Summary:
The new effect scales windows as they appear and disappear.

As the the most of window animation effects, it is a monolithic effect,
i.e., if you enable scale effect, it will animate *both* the appearing and
disappearing.

The main difference between the Scale effect and the Scale in effect is
that the Scale in effect only animates windows as they appear. There is
no corresponding "the Scale out" effect, which is odd. Other points that
differentiate the Scale effect from the Scale in effect:

• it is more subtle;
• it doesn't animate the log out screen;
• it doesn't conflict with the Fade effect, etc.

... and overall, the Scale effect supersedes the Scale in effect.

Window open animation.

Window close animation.

KCM.

Test Plan:

• Enabled this effect
• Opened/closed System Settings

Reviewers: KWin, Plasma, VDG, davidedmundson

Reviewed By: KWin, Plasma, VDG, davidedmundson

Subscribers: ngraham, davidedmundson, fvogt, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D13461