Add "force_store_value" property. If set, store, for example, calculated value as owner values.

So calculated value are calculate only one time.

tiramisu/value.py

@@ -146,6 +146,9 @@ class Values(object):
         if validate and not opt.validate(value, self.context, setting.has_property('validator')):
             raise ValueError(_('invalid calculated value returned'
                              ' for option {0}: {1}').format(opt._name, value))
+        if self.is_default_owner(opt) and \
+                setting.has_property('auto_store_value', opt, False):
+            self.setitem(opt, value)
         return value
 
     def __setitem__(self, opt, value):