Allow multiple analog waveforms to share a single WaveformArea

[azonenberg]

The current WaveformArea class allows arbitrarily many digital/protocol overlays but only supports a single analog waveform as the primary channel.

For viewing differential signals etc, it can be handy to see multiple signals in a single plot.

[azonenberg]

Works just fine in ngscopeclient, no plans to backport to glscopeclient.