improve scanning behavior

[Marzona]

an improvement on the scanning behavior could be to tune a frequency, probe it a few times before making the choice of bookmarking it or not. The problem is to define how much to wait, the "delay" value captured by the UI was inserted for allowing the User to listen and to be able to stop the scanning task if something interesting is found. We could use the same value meaning the tool captures the signal level every second "delay" times. Given the delay option is expressed in seconds, we prove once per second (approximately) for the given amount of seconds. If at least one time the signal level is high enough the frequency is going to be added in the bookmarks.