Resolves #96

Adds capability to disable spectral display.

Adds right-click context
menu items to disable display and/or reenable display by selecting
another tuner.

Adds view menu items to disable spectral display or show a tuner.

Persists spectral display enabled state across application run sessions.

src/main/java/io/github/dsheirer/spectrum/SpectrumPanel.java

@@ -327,8 +327,7 @@ public class SpectrumPanel extends JPanel implements DFTResultsListener, Setting
      */
     public void clearSpectrum()
     {
-        mDisplayFFTBins = null;
-
+        Arrays.fill(mDisplayFFTBins, 0.0f);
         repaint();
     }