ENH: Change Montage/Trace dynamically within browser

[nmarkowitz]

Describe the problem

In many instances it's useful to view the data trace in a few different ways such as with different references. It's common to view data trace using different references especially both for researchers and clinicians.

Describe your solution

When calling the plot() function allow additional kwargs or args that specify another object or parameters for that object. For instance:
raw.plot(additional_data=[raw2,raw_avg_ref, {'data': raw, 'callback': myfunc}])

Describe possible alternatives

If this can't be done then allow multiple instances of the databrowser to be open and sync them so scaling and time are the same when changed.

Additional context

Related issues:
mne-tools/mne-python#12409
#248
#133