XML files is handled as an uneditble attachments in the current 

version.

I implemented a new class XMLReviewUI that

enhence Review board's ability to process XML file.
Features:

- Smart indentation for the file

- Diff for the file will be shown in render and source format
Known issues:

- Due to the parser library issue, the XML Render cannot render

  the text of an element that are after a child element.

  (e.g. <parent>

          <child>

          </child>

          info

        </parent> )

   info cannot be render in the example above.

Added and ran unit test with:

 - Properly formatted XML files (attributes/children).

 - Handle Highly-nested XML files.
Tested the development environment with:

 - File diff system.

 - Properly formatted XML files (attributes/children).

 - Handle Highly-nested XML files.