[CForms] Field definitions aren't contextualizable

[Ugo Cei]

In trying to implement a CAPTCHA validator for CForms, I found out that 
I needed to to store an attribute in the session from a field definition 
 builder and I discovered that even if my class extending 
AbstractDatatypeWidgetDefinitionBuilder implemented Contextualizable, 
its contextualize method was never called.

After a little debugging, I discovered that the DefaultFormManager 
instantiates a SimpleComponentSelector directly but does not 
contextualize it. So, the SimpleComponentSelector cannot contextualize 
the widget builders that it creates in turn.

OK, to make it short, I locally did a quick fix (against 2.1.8-dev):
Index: src/blocks/forms/java/org/apache/cocoon/forms/DefaultFormManager.java
===================================================================
--- 
src/blocks/forms/java/org/apache/cocoon/forms/DefaultFormManager.java 
   (revision 170351)
+++ 
src/blocks/forms/java/org/apache/cocoon/forms/DefaultFormManager.java 
   (working copy)
@@ -104,6 +104,7 @@
                     manager.release(service);
             }
         });
+        widgetDefinitionBuilderSelector.contextualize(avalonContext);

widgetDefinitionBuilderSelector.configure(configuration.getChild("widgets"));
     }
I'm not sure this is the right thing to do. Would someone who is more 
knowledgeable of CForms internals please review this, so that I can 
apply it?

        Thanks a lot,
		Ugo