D8345: [QPA] Implement Screen on top of internal Screens API

[Martin Flöser]

graesslin added a dependency: D8344: Add physicalSize to the Screens API.

REPOSITORY
  R108 KWin

REVISION DETAIL
  https://phabricator.kde.org/D8345

To: graesslin, #kwin, #plasma
Cc: plasma-devel, kwin, bwowk, ZrenBot, progwolff, lesliezhai, ali-mohamed, 
hardening, jensreuterberg, abetts, sebas, apol, mart