Summary:
Done primarily for XWayland which for legacy reasons doesn't assume the
logical size of a display is pixelSize / outputScale. Meaning xwayland
windows that position themselves are wrong in a scaled environment.

It also allows the possibility for us to support fractional scaling
whilst keeping wl_output::scale as an integer.

The protocol is a bit odd as it operates via the FooManager + Foo
pattern rather than using globals like Output so I've wrapped it so it
behaves more like globals.

Test Plan: #plasma

Reviewers: romangg

Subscribers: #frameworks

Tags: #frameworks

Maniphest Tasks: T8501

Differential Revision: https://phabricator.kde.org/D12235