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:
{F6584647}