ENH: coreg legend

[GuillaumeFavelier]

This PR adds a legend to the coreg app. It helps make the association fiducial <> color:

It's an item of #8833

[GuillaumeFavelier]

I pushed a commit to adjust the size so that it is visible with the notebook 3d backend too. Now it's 10% of the scene:

notebook	pyvistaqt

[GuillaumeFavelier]

This is ready for reviews from my end @agramfort, @larsoner

[agramfort]

can you avoid redefining here the colors of fiducials and define each color in only one place? besides can you use (0, 1, 0) for nasion in legend so it's more readable.

[GuillaumeFavelier]

@agramfort I reused the values in DEFAULTS['coreg']:

[GuillaumeFavelier]

I pushed a commit to fix:

E   ValueError: 
E   	Invalid color input: ([255.   0.   0.])
E   	Must be string, rgb list, or hex color string.  For example:
E   		color='white'
E   		color='w'
E   		color=[1, 1, 1]
E   		color='#FFFFFF'

[GuillaumeFavelier]

Thank you @agramfort !