[plugins/qpa] Properly clean up the created KWayland::Client::Outputs

On build.kde.org the autotests started to crash on tear down due to a
newer Wayland library. The reason is that the KWayland::Client::Outputs
are destroyed after the internal Wayland connection is destroyed.

This change parents the created Outputs to the Registry like the other
objects. To ensure that the KWin::QPA::Screen doesn't have a problem
with that, it is changed to a QPointer - nullptr checks are already in
place.

Hopefully that will fix the crashes on build.kde.org, but there is a
chance that more errors are hidden.