Make time axis reflect elapsed time since start of recording

When recording signals, the time axis now shows the time since
start of recording, rather than always showing zero to the
selected window length.  (Not currently compatible with the
Overlay Traces feature, which still exhibits the old behavior.)
Revert delay added in StartADC

The "Wait(0.1)" added in 3.4.6 seemed to cause issues
for the timescale feature (the starting time for the
display would be a number of unknown origin rather
than simply zero).