Add threshold properties, methods, and signals

[audse]

Attribute class

• Add properties enable_thresholds (bit flags), low_threshold, high_threshold, critically_low_threshold, and critically_high_threshold

AbilitySystem class

• Add signals attribute_value_low, attribute_value_high, attribute_value_critically_low, and attribute_value_critically_high
• Add methods is_attribute_value_low, is_attribute_value_high, is_attribute_value_critically_low, and is_attribute_value_critically_high, is_attribute_value_ok