Sorting Actionable from most urgent due dates

Related to getting-things-gnome#859

Current sorting in Actionable view starts with no date tasks and most urgent dates go to the end, which means `today` might not be even visible. This simple patch reverses the sorting order of due date, so it starts with `today` tasks.

GTG/gtk/browser/treeview_factory.py

@@ -163,7 +163,7 @@ def sort_by_duedate(self, task1, task2, order):
             t1 = task1.get_due_date_constraint()
         if t2 == Date.no_date():
             t2 = task2.get_due_date_constraint()
-        return self.__date_comp_continue(task1, task2, order, t1, t2)
+        return self.__date_comp_continue(task1, task2, order, t2, t1)
 
     def sort_by_closeddate(self, task1, task2, order):
         t1 = task1.get_closed_date()