Describe the new feature or enhancement

Proposed here: https://github.com/mne-tools/mne-python/wiki/GSoC-Ideas#2-event-system-for-brain-and-evoked-plots

We have an excellent 3D viewer for brain activations, and multiple ways of viewing evoked data (plot_topo, plot_topomap, plot_joint, etc.). We want to integrate these so that you can, for example, click on a time point in a Brain and have the evoked.plot_topomap update. This should be done using callbacks to allow for customizability.

Describe your proposed implementation

This is too long to fit in an issue description. See this gist: https://gist.github.com/wmvanvliet/fa0b0894d6acffc5fce1461e3a94ae4c

Describe possible alternatives

There are many possible alternatives to this API. For the current one, I aimed for minimal impact on existing code. To not enforce the usage of classes or wrapping matplotlib Figure objects in any way.

Additional context

No response