Update SurfaceInterface outputs when an output global gets destroyed

Summary:
As per existing TODO.

A new signal is added on Global to emit so we can process the result
whist we still have a valid object. The name is overly explicit to try
and logically separate it from QObject::destroyed().

Test Plan: Updated existing unit test.

Reviewers: Plasma, graesslin

Reviewed By: Plasma, graesslin

Subscribers: graesslin, anthonyfieroni, plasma-devel, Frameworks

Tags: Frameworks, Plasma on Wayland

Differential Revision: https://phabricator.kde.org/D7531