[kcmkwin] Properly load effects
EffectsModel has to communicate with KWin in order to receive the
actual value of SupportedRole. So, in theory the model should notify
about loaded effects after receiving response from KWin, but that's
not the case.
Test Plan: Desktop Effects KCM no longer flashes when resetting changes.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Differential Revision: https://phabricator.kde.org/D18728