Documentation on future shape drawing features

[emmanuelle]

In python/shapes, add

• 1 example with drawing buttons in the modebar, dragmode selected to be draw-rectangle, and "click and drag here to draw rectangle" text annotation on an otherwise blank figure
• 1 example showing how to use the newshape attribute (on a figure with already a shape with the same attributes, to show that shape and newshape have a similar API).
• 1 example showing how to make a figure editable (useful with and without drawing shapes).
• mention that drawing a shape triggers a relayoutData event and link to the dash-docs page on interactive graphing.
• plotlywidget example, for example two subplots, one with an image and the other with the image histogram, which is updated when a line is drawn. (will require animated gif)

In python/configuration-options/, add example with drawing buttons in modebar, and link to shapes tutorial.

in imshow and layout image tutorial, mention the possibility of shape drawing.

[gvwilson]

@LiamConnors is this still required?