The Plug-In samples include a basic XML editor:
Plug-in Development/Plug-in Project.
XEdit for Name
Default values for Plug-in Project Structure.
Create a plug-in project using a code generation wizard and the Plug-in with an editor options.
Next and then the Finish option on the next screen Simple Plug-in Content;
Use a plug-in wizard to export it as a tar.gz file
Untar the file (in the eclipse directory) and copy the result to the plugins directory
After all that, it's a plain text editor with syntax coloring