PopUpManager should support constraints and percent sizing

Currently, PopUpManager assumes that pop-up components determine their own position and size. Users can set these properties on their own or they also have access to a convenient centerPopUp() method on PopUpManager.

Because pop-ups are not in a layout (instead, they are parented to the SystemManager), they cannot use constraint-based positioning/sizing or percent-based sizing.

This enhancement calls for updates to PopUpManager.addPopUp() to look for constraints and percent sizes and implement a subset of the logic found in BasicLayout. Size and position should be determined by either (1) the top level system manager screen bounds or (2) SystemManager.getVisibleApplicationRect() as currently implemented in centerPopUp().