Replace the context menu library

[julienw]

The context menu library we're using is now unmaintained.

https://www.npmjs.com/package/@szhsin/react-menu might be a good replacement. But we need to test it.

We have some difficult use cases, because we generate some context menus on the fly because of performance reasons (which may not be necessary anymore, possibly?), and there should be no flickering. Also we have different context menus for the same DOM element (think: marker chart, with one canvas but many markers).

Some difficult use cases are:

• right click somewhere in a panel => the menu opens for that element
• right click somewhere else in a panel => the menu should open for this new element, and everything works for this new element. Test this in the call tree and and canvas-based charts.

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