Make KCModuleQml conform to the defaulted() signal

Summary:
This new signal got introduced in KCModule, so we need to emit it at the
right time depending on the ConfigModule state.

This depends on: https://phabricator.kde.org/D25069

Reviewers: Plasma, Frameworks, mart, davidedmundson, dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25070