[kcmkwin] Move effects model into a shared library

Summary:
The main motivation for moving it into a shared library is to share the
model between Desktop Effects and Virtual Desktops KCM.

The extracted model is quite the same as the one in Desktop Effects KCM,
except some minor changes, e.g. rename loadEffect and syncConfig to more
convenient names, add comments, some whitespace changes, fix coding
style in some parts, etc.

Test Plan: effectModelTest passes, Desktop Effects KCM works.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: hein, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17765