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.