Add Overlay Traces feature.

Request from Michelle Day and others.  Instead of erasing trace each time
it reaches right edge of scope, make previous traces remain on the display
in a "dimmer" color (Aqua, by analogy with Event Analysis window).
Fix bug in Overlay Traces making axis label show 0 to 1000s

When Overlay Traces is selected and Record Signals window is reopened,
axis label would show 0 to 1000s regardless of window length selected.
(Traces were displaying properly, it was just an axis label problem.)
The cause was ResetDisplays not being called (Overlay Traces suppresses
ResetDisplays in order to work); the fix has ResetDisplays called by
default in FormShow.
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.