[effects] Use arg="true" in the kcfg files

Summary:
By changing all kcfg to have arg="true" we can pass in the same
KSharedConfigPtr into all effects. This allows to have fake config in
the tests and in the planned effect demo mode.

Also it means that we don't have to hardcode the name kwinrc into the
files. In the configs - where we cannot access the effectshandler - we
use the define KWIN_CONFIG which gets generated based on the compile
time arguments.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3571