Add multi-channel plotting + editing

[rmarkello]

It would be great to have the ability to synchronize plots (including editing functionality) across different physiological modalities.

Ideally plots would all share an x-axis (e.g., time) and would be stacked vertically. Scrolling through one would scroll through all of them (though, of course, editing the peak detection on each should be separate!).

This will involve some thinking about how to organize the data such that the provenance info can be saved independently for each modality!