strx-mon: Implement record button functionality.
strx: Make I/Q recorder unbuffered to avoid sudden UI freezes.
strx-mon: Trigger UI update when channel is swapped.

We reset the callback counter every time the CHAN button
is pressed. This will force an update the next time the
callback is executed.
strx-mon: Implement filter cutoff control.
strx-mon: Only update filter offset if new value is different from old one.