Augmenting of Spoon model

[pvojtechovsky]

In #2762 and #2734 there would be nice to be able to extend/augment Spoon model.

What about a new CtElement method

class CtElement {
  ...
  /** gets or creates an extension of type `type` of this element */
  <T> T adaptTo(Class<T> type);
}

We would also need

interface AdapterFactory {
   boolean canAdaptToType(Class type);
   <T> T createAdapter(CtElement element, Class type);
}

and

class Factory {
...
void registerAdapterFactory(AdapterFactory adapterFactory);

then CtElementImpl#adaptTo(Class) would look like this

class CtElementImpl {
...
/** gets or creates an extension of type `type` of this element */
<T> T adaptTo(Class<T> type) {
   T adapter = (T) metadata.get(type);
  if (adapter == null) {
    adapter = getFactory().getAdapterFactory(type).createAdapter(this);
    metadata.put(type, adapter)
  }
  return adapter;
}

WDYT?

[monperrus]

cleaning pass, closing stale issues :-)