oscplot.c: Add an info tab with Sample Rates of ADC devices

Signed-off-by: Dan Nechita <dan.nechita@analog.com>
plugins/dac_data_manager.c: Fix the retrieving of widgets or iio_widgets
Tones 0, 1, 2, 3, ... were being compared against the list of Tones 1, 2, 3, 4, ...

Signed-off-by: Dan Nechita <dan.nechita@analog.com>
plugins/fmcomms1.c: Update DDS tone freq after a DAC sampling freq change
The DDS frequency attributes change due to a change of the DDS input clock (DAC sampling freq).
This fix allows the tone freq spinbuttons to maintain their initial values, prior to the DAC sampling freq change.

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