ENH: support multiple simultaneous overlays in Brain.add_data

[payamsash]

Reference issue (if any)

Closes #11503.

What does this implement/fix?

The rendering side is already ready (#13970), so it can keep multiple named overlays together. This PR is on the Brain side so it can track more than one dataset at a time, and connecting remove_existing=False to actually work. so WDYT about these steps @larsoner?

1. modify self._data to index datasets by key instead of overwriting, so multiple calls to add_data(..., key="stats") and add_data(..., key="data") can happen.
2. connect remove_existing=False to call LayeredMesh.add_overlay with a new key.
3. what to do about colorbar? let's just show the last added overlay colorbar (for now).

[larsoner]

Yes @payamsash that sounds good to me!

Related to:

what to do about colorbar? let's just show the last added overlay colorbar (for now).

We also have an issue with interactivity, in that the Brain interface can have widgets that control colorbar limits for example. So we'll want to add a "data key" (maybe there is some better name?) drop-down there that allows controlling the limits for the given key / set of data, and we'll need some private var to track the currently "active" (meaning: colorbar is the one shown, and the limits / smoothing etc. changed by GUI interaction) data key.

[payamsash]

for the interactivity I will open another PR (but probably not soon), as I now have a good view how it works, but I think its quite challenging (at least compared to this one).

[larsoner]

FYI if you pre-commit install --install-hooks locally then run pre-commit -a it'll initialize the pre-commit envs for you, then when you try to commit it will autofix locally (and then if you try to commit again, it should succeed assuming your pre-commit issues were just something ruff could fix)

In the meantime I've started CircleCI and will review

[larsoner]

Okay I get the architecture here now. It makes sense. But I have a slightly different proposal that will help minimize diff, and thus help preserve blame and reduce review burden. Instead of what is currently implemented:

1. Modify self._data to be the dict-of-dicts storing all datasets
2. Add self._active_data_key to track what the "active" key is
3. Add self._active_data as a @property that wraps to self._data[self._active_data_key]

How about

1. Add self._all_data to be the dict-of-dicts storing all datasets
3. Add self._data as a @property that wraps to self._all_data[self._active_data_key]

It makes the code naming slightly worse, but it should make the diff much smaller, which helps maintainability (because of how blame, mostly).

[larsoner]

@payamsash I am not notified of new commits so if you need re-review feel free to ping me directly!

[larsoner]

Changes LGTM, marking for merge-when-green -- thanks in advance @payamsash !