The tree editor will be customizable by modules containing 
icons and customizers for a particular doctype.  The 
customization will be done declaratively.

For example, if you wanted to add an editor for web.xml 
files (doctype web-app), you could write a module that 
contained icons for the <servlet>, <init-param>, and 
<context-param> elements.  And you could implement a 
customizer for <servlet> that, say, browsed the classes in 
your project.  Your module would have a file that looks 
something like:

<doctype name="web-app">
  <element name="servlet" iconbase="/path/to/servletIcon">
    <customizer class="com.foo.ServletTagCustomizer">
      <property name="foo" value="bar"/>
    </customizer>
  </element>
  <element name="init-param" 
iconbase="/path/to/initparamIcon"/>
  <element name="context-param" 
iconbase="/path/to/contextParamIcon"/>
</doctype>