[kcmkwin] Split out Desktop Effects KCM

Summary:
A while ago desktop effects and compositing settings lived under the
same roof

{F6584639}

{F6584642}

but time has passed and now those two have their own kcms. This causes
some issues:

* for newcomers it's harder to find code of the Desktop Effects KCM;
* git history doesn't look good, e.g. "[kcmkwin/compositing] Add some
  bugs to Desktop Effects KCM to fix later";
* in general, the mix of two doesn't look good in the code.

This change splits out the Desktop Effects KCM. Unfortunately, in order
to have more nicer code I had to refactor EffectModel a little bit.

Before:

{F6584669}

After:

{F6587570}

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, ltoscano, mart, ngraham, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18703