ENH: add overlay Brain GUI

[payamsash]

Reference issue (if any)

following #13995 and "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."

What does this implement/fix?

Add a box next to the "Color Limits", (it should be hidden when there's only one overlay), switching it should update _active_data_key and refresh the fmin/fmid/fmax/smoothing sliders. so when add_data(remove_existing=False) is called after the viewer is already open, the box should update its items. but we'd need to add a small set_items(items) method to _AbstractWdgt and put it in both _qt.py and _notebook.py, because now the set_range calls setRange(min, max) which is numeric-only and not compatible with QComboBox.

not sure if there's a simpler way? @larsoner

Additional information

I'll probably use Claude AI for code review.

[larsoner]

Feel free to ping me when to look or try!

Bonus points if you update an example to show this... maybe use multiple (dSPM, sLORETA, eLORETA?) solvers in https://mne.tools/stable/auto_tutorials/inverse/30_mne_dspm_loreta.html and overlay them with some suitable alpha and different colormaps (red/green/blue?). It could make for a cool visualization at the end of the example, and then easy to test interactively. But not obligatory if you have other ideas!