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

从链接的 dev-tooltip 分支、其 README 和在线示例开始，然后结合现有的 annotations 和 hovertemplate 行为，审查提议的 modebar 按钮。Dash 演示位于 demo/dash/dash_app_with_tooltip.py。当 tooltip annotation 的行为、自定义、编辑、拖动和删除均按描述正常工作，并且仍与现有的 plotly.js 用法兼容时，即视为完成。