add tooltip annotation feature

Hi,

I would like to propose a tooltip feature.

When exploring data and presenting our findings, it is important to be able to interactively annotate data points. Most popular data analysis software allow to do that.

I wrote a code to introduce this functionality (found in this branch). The tooltip function can be activated by a new modebar button. It allows to add an annotation to every clicked point. By default, tooltips contain x and y coordinates. They can be customized to contain any additional data attached to the trace or points, which makes it a powerful tool for data exploration and presentation.

When a plot is created with editable: true, the tooltips can be dragged around for placement or deleted interactively. Their text can also be edited. To delete a tooltip, click on its text and delete it.

Tooltips can be customized with an optional tooltiptemplate (leverages existing hovertemplate code) and tooltip annotation options (leverages existing annotations code)

Live example

The JS version is already compatible with Dash when a figure is created with a dictionary passed to dcc.Graph (Dash demo) but not yet with Plotly.py

Comienza con la rama dev-tooltip enlazada, su README y el ejemplo en vivo; después revisa el botón modebar propuesto junto con el comportamiento existente de annotations y hovertemplate. La demo de Dash está en demo/dash/dash_app_with_tooltip.py. Se considera terminado cuando el comportamiento de las anotaciones de tooltip, su personalización, edición, arrastre y eliminación funcionan como se describe y siguen siendo compatibles con el uso existente de plotly.js.