Merge branch 'overlay_traces' into recadc_timescale_and_overlay_traces
Make recadc_timescale work with overlay_traces

The change to recadc_timescale was incompatible with overlay_traces (the
former relies on the ScopeDisplay's "Invalidate" function which redraws
the axes; Overlay Traces suppresses Invalidate to keep the previous
traces displayed).  This fix allows both to work together.
Fix bug in recadc_timescale_and_overlay_traces

Bug found by S. Graves caused WinFluor to crash in
Record Signals window when stimulus Start and Stop
buttons were pushed (not necessarily when recording).
Comment for line in question claims "So OverlayTraces
works correctly", yet removing line still allows
OverlayTraces to work.