Added suppport for automatic creation of trigger callbacks.  Any object
with a _BIND_TRIGGER attribute will try to create an mGui callback which
fires the named attribute automatically.
cleaned up comment
Support for automatic binding triggers: several classes now have
annotations allowing Bindings to create a trigger callback at creation
time.

Also, RadioCollection and IconTextRadioCollection are now context
managers, so that they can create automatic trigger callbacks for their
children
radiobuttons update all available bindings, not just the first