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.