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

[christinerogers]

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 -

[Andesha]

Named as per EEGLAB terminology.