Use time instance to format time widget in tree

The value of a time widget can be a datetime instance.
The changeset acffa8c5cf89 changed the strftime by datetime_strftime but it can
not format time instance so the value must be converted into time and use
strftime.

issue5930
review33711002
(grafted from 54afc9d4eea99d82bff7d4196a7c9e885f764cb8)

tryton/gui/window/view_form/view/list_gtk/widget.py

@@ -1,6 +1,6 @@
 # This file is part of Tryton.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
-
+import datetime
 import os
 import tempfile
 import gtk
@@ -387,7 +387,9 @@ def get_textual_value(self, record):
             return ''
         value = record[self.attrs['name']].get_client(record)
         if value is not None:
-            return datetime_strftime(value, self.renderer.props.format)
+            if isinstance(value, datetime.datetime):
+                value = value.time()
+            return value.strftime(self.renderer.props.format)
         else:
             return ''