Fix double context menu in Terminal tab

This was introduced in 1f71ccd4 because the VTE widget's existing
button-press handler didn't return `TRUE` and so the event propogated
up to the notebook showing both of their context menus.

src/vte.c

@@ -536,6 +536,7 @@ static gboolean vte_button_pressed(GtkWidget *widget, GdkEventButton *event, gpo
 	{
 		gtk_widget_grab_focus(vc->vte);
 		gtk_menu_popup(GTK_MENU(vc->menu), NULL, NULL, NULL, NULL, event->button, event->time);
+		return TRUE;
 	}
 	else if (event->button == 2)
 	{