support "sensitivity" and "timebase" style scaling in raw data browsers

[drammock]

From the forum: https://mne.discourse.group/t/scaling-sensitivity-uv-mm/5079

The idea is that the raw data browser should have a way to take into account the DPI of the display when setting the signal scaling (both amplitude scaling, and when setting the duration of signal shown onscreen). This is apparently important for clinical practice, where certain waveform patterns are diagnostically useful, and visual recognition of those patterns is dependent on knowing the scaling (e.g., "if I see such-and-such pattern when viewing at 7mm/uV and 30mm/sec, I know it is was a seizure")

The linked forum issue has details about which scaling values are diagnostically useful.