#1 Improve the API for default interface implementations

[Henrik Kaipe]

The purpose of this API is right now described at "http://callbackparams.org/guide.html#defaults"

The current API itself is not worth describing, however. An idea for a better API is to have the callback-interface specify an inner class that implements CallbackFactory and have that factory produce an instance of the callback-interface at hand; or indicate there will not be any default available by producing null or an instance of anything that does not implement the callback-interface at hand.

The above CallbackFactory-instance would perhaps receive information on the callback-record element at hand as well as the callback-reference (e.g. the callback-method argument or callback-field) through a thread-local static API.