Adds a arc to the current page. The arc is drawn in the current
color at the current width
and with the current options.
The arc is fixed at the center coordinate and can have different
horizontal and vertical radii. Drawing starts at the start angle
and the arc is swept out until the end angle is reached. Angles
are measured anti-clockwise with zero at three o'clock.
The AddArc function returns the Object ID of the newly added Graphic
Object.