Use KWin's KSharedConfigPtr in ScriptedEffect

Summary:
So far ScriptedEffect used EffectsHandler::effectConfig to get the
KConfigGroup for the ScriptedEffect. This has the disadvantage that the
config file name is hardcoded to kwinrc in EffectsHandler::effectConfig.

Inside KWin a KSharedConfigPtr is used which can point to somwhere else
than kwinrc. If that was the case the ScriptedEffects were not able to
pick up this customized config and instead continued to read values from
kwinrc.

With this change the ScriptedEffects use the KSharedConfigPtr provided
by KWin. Thus in e.g. autotests we can use the general way to configure
the effects and don't need to write to the config.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3383