Support dynamic output enabling/disabling from KScreen

Summary:
We need to keep the DrmOutput object to still have the
WaylandOutputDevice.

Screens currently start off enabled as before.

In order to keep KWin to have a correct index based list of screens we
need to store a second vector of currently enabled outputs for the
screens interface.

Test Plan:
Had dual screens.
Disabled/Enabled each one through the kscreen KCM

Reviewers: #plasma, graesslin

Reviewed By: #plasma, graesslin

Subscribers: ngraham, luebking, broulik, graesslin, plasma-devel, kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D8796