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