fmcomms2.glade: Put back the action widget of the calibration dialog
Glade manages to delete this widget every time I modify and save this file. I'm using 3.12.1 version which is designed for GTK+3. I should consider using Glade for GTK+2 (version 3.8).

Signed-off-by: Dan Nechita <dan.nechita@analog.com>
plugins/fmcomms1.c: Run a FFT plot in the calibration dialog background
The fmcomms1 calibration requires the iio-oscilloscope to have a complex FFT plot running with image markers set.

Signed-off-by: Dan Nechita <dan.nechita@analog.com>