What is the best way to create a menuitem (for the Gtk.MenuBar) that should open the default browser with a new tab and loading an URL? 
Is it possible to do that in Glade directly or do I need to create that function in the program code itself? Is there a preferred way in Python 3 to do that?
After a lot of searching for a Glade-only solution, I think that Gtk.Menuitem doesn't have a URL-open option. I now just defined on_menuitem_clicked-function that uses:
webbrowser.open_new_tab()
from the standard library.
Link to website in Gtk.MenuBar using Glade