[Vis] New plot: Show Variance across all channels collapsed/stacked on a single axis

per 18 Oct techComm meeting

View overall signal variance at a glance:
plot all channels laid on top of each other.

- new plot in a signal file panel, can be pre-computed, no time scroll (pre-computed, maybe you can zoom)
- mouseover on a signal line should show channel name, time and value (TBC)
- click to expand or maybe pop out (not necessarily visible by default due to performance cost)

Useful for:
- allows users to see e.g. a lot of muscle activity

Example from Tyler's MNE experimental plog -