[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