[ENH] Add fig.mne container for colorbar

[ruuskas]

This fixes Issue 262 in mne_connectivity.

I added a fig.mne SimpleNamespace, which holds the Colorbar for a circular plot. This allows users to directly access the Colorbar after creating a figure using mne_connectivity.viz.plot_connectivity_circle, which calls mne.viz.circle._plot_connectivity_circle.

I will add something in the docstring of plot_connectivity_circle to describe this functionality and modify one of the examples to use it.

[drammock]

I don't want to delay this PR as it's quite straightforward and effective... but we really ought to have a larger discussion/decision about how items in that namespace are organized (flat list or heirarchy) and how they are named. I don't want it to become an unstructured dumping ground for "anything the user might want access to after the figure is created". Since we're planning a release later this week, I'm tempted to delay merging this just a few days (until after the release), so the namespace doesn't become public until we've had a chance to put some guiderails on it. @larsoner WDYT?

[larsoner]

Sounds reasonable to me!

[larsoner]

FYI for future reference I think this would have been equivalent and shorter!

Suggested change

	Add ``fig.mne`` container for :class:`Colorbar <matplotlib.colorbar.Colorbar>` in :func:`plot_connectivity_circle <mne_connectivity.viz.plot_connectivity_circle>` to allow users to access it directly, by `Santeri Ruuskanen`_.
	Add ``fig.mne`` container for :class:`~matplotlib.colorbar.Colorbar` in :func:`plot_connectivity_circle <mne_connectivity.viz.plot_connectivity_circle>` to allow users to access it directly, by `Santeri Ruuskanen`_.

[larsoner]

Thanks @ruuskas

[larsoner]

Uggghhh forgot that we wanted this for 1.10 not 1.9. I'll revert quickly and then we'll bring it back in for 1.10

[larsoner]

Back in main in fa28abf

https://mne.tools/dev/changes/devel.html