Fix applets not being added on service restart

Summary:
Applet::destroy doesn't delete the applet from Containment::applets in
the same event. Potentially a dbus activated service being restarted can
be added in this time, causing us to try adding a new applet with the
same ID before the first object is deleted.

Reviewers: Plasma, mart

Reviewed By: mart

Subscribers: mart, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2703