iio_widget lib: Add implementation for spinbuttons with progress bar.
    - Spinbuttons with progress bar are used in situations where you don't want the value of the button to be written right away, but instead it waits for a short period of time.
    - This is helpful when you deal with attributes that can take only specific values and automatically they change user's input value to the nearest value that the attribute can take.
    - Whithout it, you will not be able to spin up or down the spinbutton
Fmcomms2 Plugin: Make it signal based.