Add ability for define tag to append to an existing definition

In Facelets one can use a ui:define tag to define content in a template client. This named content can be inserted into a template by using ui:insert.

When the template client that defines the content is itself a template, its template client may also want to contribute to this content, typically by appending to it. This is currently not possible. The template needs to use a new ui:insert tag with a new name.

E.g.

template.xml

<f:view>            
    <ui:insert name="links"/>        
</f:view>

general.xml

<ui:composition template="template.xhtml">     
   <ui:define name="links">
         <h:link ... />
         <ui:insert name="links1"/>    
   </ui:define>        
 </ui:composition>

orders.xml

<ui:composition template="general.xhtml">     
   <ui:define name="links1">
      <h:link ... />
      <ui:insert name="links2"/>     
   </ui:define>        
 </ui:composition>

etc

Instead of manually 'chaining' defines and inserts in this way, Facelets could define tags or attributes for contributing to existing definitions. E.g.:

general.xml

<ui:composition template="template.xhtml">     
   <ui:append name="links">
      <h:link ... />    
   </ui:append>        
 </ui:composition>

orders.xml

<ui:composition template="general.xhtml">     
   <ui:append name="links">
      <h:link ... />    
   </ui:append>        
 </ui:composition>

Instead of introducing a new ui:append tag, a contribute="append" on ui:define could be used instead. Next to appending, prepending could be considered via a ui:contribute tag or contribute="prepend" attribute.

The issue names no implementation files or tests. Start by tracing existing ui:define and ui:insert handling in Facelets, then settle whether ui:append or a contribute attribute is the intended API. Done means nested templates can append to an existing definition without manual chaining.