[kcmkwin] Add requestConfigure to effects model

Summary:
The main motivation for adding this method is to avoid code duplication.
Both virtual desktops kcm and desktop effects kcm have their own logic
to create configuration dialogs for effects.

On the bright side, if we add support for declarative effect kcms, we
will need to change only this method.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18704