New squelch behavior and fixes

Squelch value now goes from -90 to +20 and it's directly compared against the max_db parameter returned from each freq scanned by the radio subsystem, with no adjusts or manipulation (you adjust the number as will be used).

Less squelch means weaker signals will trigger it. (as expected).

There was a tiny cosmetic bug when you deleted a frequency from the scanning memory: The description was not erased from screen and you could see it while the scan did not resume.

There was another bug on the pause button: If you asked for another manual scan range when paused, the button kept the text "RESUME"  (its text was not reset to "PAUSE" again).