GUI requests: tiny button to clear specan

[TadasSu]

1. it would be nice to have one tiny button anywhere in specan tab to clear it.
   No I toggle 'Max ever rssi' to have empty graph without stopping specan.

[trollwookiee]

Edited and splitted this request in :

• #72
• #73
• #74

For the future, please split requests or bugs into single issues

[trollwookiee]

Fixed