Implement auto-squelch.

Pressing the auto-squelch button sets the squelch level to the current
signal/noise level.

Closes csete/gqrx/#72
Use existing rx->get_signal_pwr() instead of adding a new getter.
Use clearer names and documentation.
Updated.
Use correct widget name on OSX
Revert "Merge branch 'auto-sql' into mine"

This reverts commit b98aa9956c16e3009ee347bf5b8fc0c7828c4882, reversing
changes made to c670462fdad1a4681d5ecfe387988864f3faffd8.
Merge remote-tracking branch 'upstream/master' into mine