Add signal for when annotations change

sdrgui/gui/glspectrumview.cpp

@@ -3599,6 +3599,8 @@ void GLSpectrumView::updateWaterfallMarkers()
 
 void GLSpectrumView::updateAnnotationMarkers()
 {
+    emit updateAnnotations(); // Notify other plugins we have updated annotations
+
     if (!(m_markersDisplay & SpectrumSettings::MarkersDisplayAnnotations)) {
         return;
     }

sdrgui/gui/glspectrumview.h

@@ -535,6 +535,9 @@ private slots:
 signals:
     // Emitted when user tries to scroll to frequency currently out of range
     void requestCenterFrequency(qint64 frequency);
+    // Emitted when annotations are changed
+    void updateAnnotations();
+
 };
 
 #endif // INCLUDE_GLSPECTRUMVIEW_H