Reinstate openconnect_set_external_browser_callback handling in Gtk
This reinstates commit dfbfec25 ("Add openconnect_set_external_browser_callback for auth-dialog") having fixed it not to invoke Gtk APIs from the libopenconnect I/O thread.
This reinstates commit dfbfec25 ("Add openconnect_set_external_browser_callback for auth-dialog") having fixed it not to invoke Gtk APIs from the libopenconnect I/O thread.