reset tree_states_done when setting a screen group

The O2M screen's tree_states_done property is filled with the view id, this
property is used to know when the state must be set or not. This has the effect
of not restoring the state of O2M widget when switching from a record to
another. This change fixes this behavior.

issue3609
review2791003

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

@@ -76,6 +76,9 @@ class Screen(SignalEvent):
         self.parent_name = None
         self.exclude_field = exclude_field
         self.filter_widget = None
+        self.tree_states = collections.defaultdict(
+            lambda: collections.defaultdict(lambda: None))
+        self.tree_states_done = set()
         self.__group = None
         self.new_group()
         self.__current_record = None
@@ -88,9 +91,6 @@ class Screen(SignalEvent):
         self.fields_view_tree = None
         self.order = order
         self.view_to_load = []
-        self.tree_states = collections.defaultdict(
-            lambda: collections.defaultdict(lambda: None))
-        self.tree_states_done = set()
         self.domain_parser = None
         self.pre_validate = False
         self.view_to_load = mode[:]
@@ -242,6 +242,7 @@ class Screen(SignalEvent):
             self.group.signal_unconnect(self)
             for name, field in self.group.fields.iteritems():
                 fields[name] = field.attrs
+        self.tree_states_done.clear()
         self.__group = group
         self.parent = group.parent
         self.parent_name = group.parent_name