Synchronize send to external editor buttons

Keep all buttons updated when using a multiple editor tabs mode.

rtgui/editorpanel.cc

@@ -2003,6 +2003,9 @@ void EditorPanel::sendToExternalChanged(int)
         index = -1;
     }
     options.externalEditorIndex = index;
+    if (externalEditorChangedSignal) {
+        externalEditorChangedSignal->emit();
+    }
 }
 
 void EditorPanel::sendToExternalPressed()
@@ -2069,6 +2072,23 @@ void EditorPanel::syncFileBrowser()   // synchronize filebrowser with image in E
     }
 }
 
+ExternalEditorChangedSignal * EditorPanel::getExternalEditorChangedSignal()
+{
+    return externalEditorChangedSignal;
+}
+
+void EditorPanel::setExternalEditorChangedSignal(ExternalEditorChangedSignal *signal)
+{
+    if (externalEditorChangedSignal) {
+        externalEditorChangedSignalConnection.disconnect();
+    }
+    externalEditorChangedSignal = signal;
+    if (signal) {
+        externalEditorChangedSignalConnection = signal->connect(
+                sigc::mem_fun(*this, &EditorPanel::updateExternalEditorSelection));
+    }
+}
+
 void EditorPanel::histogramProfile_toggled()
 {
     options.rtSettings.HistogramWorking = toggleHistogramProfile->get_active();
@@ -2203,6 +2223,18 @@ void EditorPanel::onAppChooserDialogResponse(int responseId)
     }
 }
 
+void EditorPanel::updateExternalEditorSelection()
+{
+    int index = send_to_external->getSelected();
+    if (index >= 0 && static_cast<unsigned>(index) == options.externalEditors.size()) {
+        index = -1;
+    }
+    if (options.externalEditorIndex != index) {
+        send_to_external->setSelected(
+            options.externalEditorIndex >= 0 ? options.externalEditorIndex : options.externalEditors.size());
+    }
+}
+
 void EditorPanel::historyBeforeLineChanged (const rtengine::procparams::ProcParams& params)
 {