A VisLayer describes one layer of data visualized by the VisManager.
Each layer has a paint() method, which is called by the VisManager, if it wants
the layer to be drawn. The layer should use the Vis.getCanvas() method and
the transformation methods of the Vis singleton to draw its content.
Additionally, each logical terminal layer should set a help string by the
setHelpOverrideString() method, to be correctly listed in the help layer
(if it is present in the VisManager).