[KCM/Effects] Allow the user to know when the settings are set to default

Summary: It works for the effect

Test Plan:
kcmshell5 kcm_kwin_effects
Change some settings, the "Restore defaults" button is enabled when the state is not the default state.
Open an effect configuration, the "Restore defaults" button is enabled when the settings are not default.

Reviewers: KWin, crossi, ervin, zzag

Reviewed By: KWin, zzag

Subscribers: zzag, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D26040