Tracks the position of a logical progress bar. A progress bar is created with start and end points, defining its length. The position of the bar may be set or stepped (incremented) directly; or else a region of the bar may be marked and a sub-bar created to occupy that region, which then has its own independent start and end points that are mapped onto the region of the parent bar. This class keeps track of progress bars nested in this way, and calculates the overall position in the top-level bar.
Public Member Functions