[core] Add validValue and errorMessage support for ChoiceParams

ChoiceParams can now also have their `validValue` and `errorMessage`
parameters.

The `validValueChanged` is also emitted whenever a parameter's value
changes. This ensures that the `validValue` parameter is always re-
evaluated when the attribute is updated, even if `validValue` itself does
not change.

meshroom/core/attribute.py

@@ -196,6 +196,7 @@ class Attribute(BaseObject):
             self.requestNodeUpdate()
 
         self.valueChanged.emit()
+        self.validValueChanged.emit()
 
     def upgradeValue(self, exportedValue):
         self._set_value(exportedValue)