KWayland::Client::Surface track output destruction

Summary:
A compositor should send left events before deleting an output; however
if it doesn't, we don't want dangly pointers in our list of outputs on
the client surface.

Test Plan: Unit test

Reviewers: Plasma, graesslin

Reviewed By: Plasma, graesslin

Subscribers: plasma-devel, Frameworks

Tags: Plasma on Wayland, Frameworks

Differential Revision: https://phabricator.kde.org/D7379