Prevent Directories tab from closing on MacOS

* The close button on custom tabs cannot be hidden on MacOS for some reason.
* Prevent the directories tab from closing if the close button was clicked by mistake

qt/tabbed_window.py

@@ -344,6 +344,11 @@ class TabBarWindow(TabWindow):
     @pyqtSlot(int)
     def onTabCloseRequested(self, index):
         current_widget = self.getWidgetAtIndex(index)
+        if isinstance(current_widget, DirectoriesDialog):
+            # On MacOS, the tab has a close button even though we explicitely
+            # set it to None in order to hide it. This should prevent
+            # the "Directories" tab from closing by mistake.
+            return
         current_widget.close()
         self.stackedWidget.removeWidget(current_widget)
         # In this case the signal will take care of the tab itself after removing the widget