Synchronize send to external editor buttons

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

rtgui/editwindow.cc

@@ -250,6 +250,7 @@ void EditWindow::addEditorPanel (EditorPanel* ep, const std::string &name)
 {
     ep->setParent (parent);
     ep->setParentWindow(this);
+    ep->setExternalEditorChangedSignal(&externalEditorChangedSignal);
 
     // construct closeable tab for the image
     Gtk::Box* hb = Gtk::manage (new Gtk::Box ());
@@ -288,6 +289,7 @@ void EditWindow::remEditorPanel (EditorPanel* ep)
         return;    // Will crash if destroyed while loading
     }
 
+    ep->setExternalEditorChangedSignal(nullptr);
     epanels.erase (ep->getFileName());
     filesEdited.erase (ep->getFileName ());
     parent->fpanel->refreshEditedState (filesEdited);