Emit 'groupHiddenChanged' signal.
Summary:
Emit 'groupHiddenChanged' when group hidden value change
Depends on D8450
Test Plan: Unit test created
Reviewers: franckarrecot, mwolff, mlaurent
Reviewed By: mwolff, mlaurent
Subscribers: ngraham, mwolff, Frameworks
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D9241