Add support for ScaledPixel

If Coord is set to ScaledPixel for any object, then all supplied pixel values should be scaled by the DPR. The demo menu now has a check box "Scale"; if this is check and DemoBoxes is run, the form will be created with Coord=ScaledPixel.

{"WC":{"ID":"F1","Properties":{"Caption":"Boxes and Lines","Coord":"ScaledPixel","Posn":[0,0],"Size":[600,400],"Type":"Form"}}}

Note that the Coord property is INHERITED by all child objects, so once the form has ScaledPixel, all children will inherit this setting if nothing else is set.

Other demos will be enhanced to use ScaledPixel once Boxes is working.