[KCM/Desktop] Port from KQuickAddons::ConfigModule to ManagedConfigModule

Test Plan:
kcmshell5 kcm_kwin_virtualdesktops
kcm works as before

Reviewers: #kwin, ervin, crossi, zzag

Reviewed By: #kwin, ervin, zzag

Subscribers: zzag, bport, crossi, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D25881