Convert to unicode the string of predefined search field

DomainParser expect to work with unicode and gettext return string encoded in
utf-8 (coding of the po file)

issue3440
review1301002

tryton/gui/window/view_form/screen/screen.py

@@ -140,7 +140,7 @@ class Screen(SignalEvent):
                         ):
                     if name not in fields:
                         fields[name] = {
-                            'string': string,
+                            'string': string.decode('utf-8'),
                             'name': name,
                             'type': type_,
                             }