provide generic event dependencyChanged

[mpo]

currently controls can already be 'dependent' on others, when those change (ie send valueChanged event) it will trigger some function to load options data based on a uri-template that is resolved with the values of those dependency-controls

This makes the dependency-mechanism only usable in combination with an AJAX setup for downloading options data.

We should decouple the two aspects via a new event dependencyChanged() that is triggered when dependency controls have changed. This event can then be handled by custom code to cover other needs.

The current uri-loading behavior should then also moved into a standard built-in handling this event when the uri-template is provided.

See also discussion in: ​http://groups.google.com/group/kauri-discuss/browse_thread/thread/d367219a53a52522

members:{ 
    "field1" : { 
          "base":"string", 
          "control":{ 
                "base":"selection-control", 
                "options": { 
                    "data": [ '1', '2', '3'] 
                } 
          } 
    }, 
    "field2": { 
        "base":"string", 
        "control": { 
                "base":"selection-control", 
                "depends": {"f1": "field1"} 
        } 
    } 
} 

in combo with 
     form.findControl("/field2").dependencyChanged( 
         function( deps, opts) { 
             if (deps.f1==1) opts.set([1], ['one']); 
             if (deps.f1==2) opts.set([1,2], ['one','two']); 
             if (deps.f1==3) opts.set([1,2,3], 'one','two','three']); 
         } 
     );