largely reverts b164e9912c9b5f9c9ae619bbd79355d317174115
which prevented the crash but due to early screen updating
causes issues with scene/overlay update on at least intel chips
BUG: 322970
FIXED-IN: 4.11
CCBUG: 322156
REVIEW: https://git.reviewboard.kde.org/r/111811/
Following the approaches of other split out functionality Screens is a
singleton class created by Workspace.
The class takes over the responsibility for:
* screenChanged signal delayed by timer
* number of screens
* geometry of given screen
* active screen
* config option for active screen follows mouse
The class contains a small abstraction layer and has a concrete subclass
wrapping around QDesktopWidget, but the idea is to go more low level and
interact with XRandR directly to get more detailed information.
All over KWin the usage from QDesktopWidget is ported over to the new
Screens class.
REVIEW: 109839
