Add extension mechanims to declare that supports name-free 'inherited'

[sjmiles]

Declare names functional properties so that a short-form of 'inherited' can be used.

dojo.declare("myClass", null, {
  method: function() {
    // .. code ... 
    this.inherited(arguments);
  }
}

Alter declare to add an 'extend' method onto declared classes such that class extensions can also use short-form 'inherited'.

myClass.extend({
  extension: function() {
    // .. code ... 
    this.inherited(arguments);
  }
});

[sjmiles]

(In [10408]) Add 'extend' method to declared classes (not instances), fixes #4348.