Allow overriding actions inside a BindingPool

Allow overriding actions inside a BindingPool

As of now, a key binding installed into a BindingPool is always there
and cannot be changed.

This is problematic for sub-classes trying to override the callback
or the action for a given key binding.

This commit adds the ability to override the closure for an existing
key binding inside a binding pool -- assumed the caller knows the
key symbol and modifiers used to install the key binding in the first
place.