Added support for circle segments in the 'Circle' shape.

Added the 'angle' attribute with corresponding 'angleDeg' function to the 'Circle' shape. The attribute defines the sweep angle for the circle segment. The circle segment will hence sweep between [0-'angle'] radians (or degrees).

The idea is to define the size of the circle segment with the 'angle' attribute and then, if desired, rotate it with already existing 'rotation' attribute.

Also added a unit test for the new functionality: 'SHAPE - add circle segment using Circle'.