Change envelope calculations for waveshow?

Related to #2008 and #2012

Now that we have multiplot display, there's less of a need for supporting stereo signals in waveshow with this quirky left/right upper/lower envelope calculation.

If we deprecate anything other than mono waveshow display, we can streamline some things in the envelope calculation. In particular, we can use max/min to calculate the envelope instead of ±max(abs).

The key benefit here is that we would see less of an abrupt transition between envelope and sample modes when zooming in, and it would put us more in line with similar plots in other software tools (eg audacity).

I'd propose that we deprecate the stereo functionality now (1.0 release) and drop it in 1.1, at which point we'd also switch over to min/max envelopes.