Use std::transform in libkwineffects implementation part

Summary:
Don't use raw loops, instead, use appropriate algorithm from STL to map
a list of toplevels to a list of EffectWindow.

Test Plan:
Tested Cover switch, Flip switch, Dialog parent, and Dim inactive effect.
They all still work.

Reviewers: KWin, graesslin

Reviewed By: KWin, graesslin

Subscribers: graesslin, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D13821