Support for custom tooltips (i.e. tippy) #2777

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What problem does this feature solve?

It's currently not possible to use custom tool tips with G6, especially in Canvas mode because most tool tips implementations require an HTML element to attach to and a hook for destroying the tool tips once they are used. These two are missing in G6 currently.

For example, we are using tippy js (https://github.com/atomiks/tippyjs) throughout our application and we'd like G6 tooltips to have the same look and feel by reusing our custom-made tooltips.

What does the proposed API look like?

A generic way to use any custom tooltips with G6. Most tooltip libraries need the following

• Expose an HTML element for tippy (or other custom tooltip libraries) to attach to (including in Canvas mode)
• Expose a hook to destroy a tooltip after it is used (could be something similar to mouseleave event?)

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