FFT: Markers are actually dB Full Scale (dBFS), not just dB.
     For an understanding of the difference:
     http://www.analog.com/static/imported-files/tutorials/MT-003.pdf

Signed-off-by: Robin Getz <robin.getz@analog.com>
DMM: Fix runtime warnings by fixing glade file

Signed-off-by: Robin.Getz <robin.getz@analog.com>