Summary:
AbstractOutput::name() behaviour is changed so that it matches the X11
behaviour, showing an identifier like "HDMI-0".

XdgOutput.name is set to this name.

XdgOutput.description is currently set to the manufacturer name and
model, but it's not exposed to Qt so we probably don't care too much.

This should fix plasmashell changing applets when switching between X11
and wayland.

Test Plan:
Relevant unit test
I still need to run it on my laptop.

Reviewers: #kwin, #plasma, zzag

Reviewed By: #kwin, #plasma, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D28230