Add physicalSize to the Screens API

Summary:
Allows to share the implementation in a better way and is a requirement
to get the Screen implementation in the QPA plugin to be based on
KWin::Screens instead of KWayland::Output.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D8344