Refactor getters in OutputScreens

Summary:
kdelibs coding style recommends to avoid abbreviations, i.e. variable
names like compMngr, errStr, and so on are bad. Unfortunately, all
getters in OutputScreens don't follow that recommendation. In general,
we could address this issue by renaming enOuts but there is a bit simpler
approach to implement most of those getters.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: apol, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D20588