Fix for failing testScriptedEffectsLoader

By changing the loading of scripted effect config to interact with
kwinApp we broke the tests which do not use a kwinApp.

This change turns the access to the KSharedConfigPtr go through the
property system, so that the tests can also install a dummy
KSharedConfigPtr. With that scripted effects loader test passes and no
longer crashes and the integration/effects tests also still pass.