Allow single frame to be held in Record Images display

In the Record Images window, a static image can be
acquired and held in the display ("pausing" live
acquisition).  This is useful when, e.g., laser imaging
and auxiliary camera imaging cannot be done simultaneously.
Use StopCamera instead of holding frame in separate buffer

John Dempster suggested using the StopCamera procedure to
freeze the frame instead of the buffer manipulation I
originally did.