Screens provides a size which is constructed from the size of
the bounding geometry of all screens and provides an overload taking
an int to return the size of a specified screen. For geometry() a new
ovload is added without an argument, which is just a convenient wrapper
for QRect(QPoint(0, 0), size()).

Both new methods are exported to effects and scripting as new
properties there called virtualScreenSize and virtualScreenGeometry.

The (virtual) size gets cached in screens and is updated whenever the
count or geometry changes.

Construction of Screens is slightly changed by moving the init code
from ctor into a virtual method init(). Reason is that we ended in
a loop with accessing the singleton pointer before it was set.

REVIEW: 116114