add listener support for points added by clicking

Add support for consumers of points added by clicking on the graph. For example, this would let the graph controller in Lab add user clicked points into a data set. For some of the RITES interactives we would like to have graphs that users draw graphs with a linked table showing the data points. This patch adds the necessary features to the grapher. 

Note that the pointListeners in here are only dispatching points added by clicking.