Introduces jQuery.PurposedCallbacks constructor

An instance of jQuery.PurposedCallbacks is a list of $.Callbacks instances, one per "purpose".
A purpose is defined by a string. When registering a callback, an associated purpose has to
be given as well.

Change-Id: I312c0d52979cb276d6ed0336c53347236369a64c
Merge "Introduces jQuery.PurposedCallbacks constructor"