Provide a way to erase the active shape via a dash callback

[archmoj]

If we expose eraseActiveShape in https://github.com/plotly/plotly.js/blob/master/src/plot_api/index.js one could use something call Plotly.deleteActiveShape(gd) to erase the active shape.

The other option is that we add newshape.index to layout (instead of gd.__fullLayout._activeShapeIndex which is internal at the moment). Then users could use if for updating the list of shapes.

cc: @tamidodo