mne.viz.Brain modernization

Describe the new feature or enhancement

working with mne.viz.Brain GUI, I think we can significantly improve it:

• colorbar currently shows numbers with no context, also we can improve the font and size
• background color can only be chosen when initiating Brain, we could also add str color options (in the docs says tuple (int, int, int))
• better Help dialog
• when an annot is selected we could click on or select a parcel and see the extract_label_time_course signal
• maybe highlighting (a) parcel(s)
• show vertex coordinate and camera orientation when hovering mouse
• better foci visualization
• better trace viewer
• most of the args in the mne.viz.Brain are missing in the GUI
• more keyboard shortcuts
• collapsable panels
• ...

I'm happy to open PRs, but I would like to first see your opinions, tnx @larsoner @drammock @wmvanvliet

Describe your proposed implementation

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Describe possible alternatives

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Additional context

No response