QCursor: Associate cursor with screen.

QCursor: Associate cursor with screen.

- Introduce cursor() accessor to QPlatformScreen.
- Remove screen member of QPlatformCursor (a
  cursor can be shared by multiple screens
  of a virtual desktop).
- Add QCursor::pos()/ QCursor::setPos() taking
  a QScreen-parameter, use primaryScreen() for
  old overloads.  QCursor::pos() can then query
  the platform cursor for the position and return
  the position even if the mouse position is outside
  the windows owned by the Qt application.
- Fix tests

Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>
Task-number: QTBUG-22457
Task-number: QTBUG-22565
Task-number: QTBUG-20753
Change-Id: Ia69f37343f95772e934eab1cd806bd54cbdbbe51
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>