Added attribute event on_change
rename remove_events in clear_events and remove_all in clear
Make Method and Attribute Value inherit from descriptors Value to remove some duplication
Move WrapCondition in module descriptors.wrappers
Value find handlers name thanks to set_handlers
Make handlers propagation coherent
Added Value.clear_all and sub methods (call_all, call...)
Added method "when" to Value to handle more easily handlers
Add a test of Value.when at Attribute level