Port away from serviceOwnerChanged signal

Summary:
Use QDBusServiceWatcher::service(Un)registered signals, since in
most cases we only need to see one or the other, rarely both and
we basically never want to execute the same code for both.

Reviewers: vkrause

Reviewed By: vkrause

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D26282