Ensure that WaylandServer::shellClientAdded only gets emitted once

Summary:
When a shell client got mapped, unmapped and mapped again we emitted
the shellClientAdded signal in WaylandServer again. This resulted in
e.g. Workspace, EffectsHandler, etc. to start managing the window again.
This can be a reason for problems we see with such windows like the
Plasma panel dialog when opened the second time.

Test Plan:
Needs extensive testing on real world system as that changes
behavior now.

Reviewers: KWin, Plasma on Wayland

Differential Revision: https://phabricator.kde.org/D1784