[T] xml patch functionality

Things to discuss:

• Supply multiple patch files?
• Add the file to patch (base file) to the patch file? E.g.:

  <OpenGeoSysProjectDiff base_file="square_1e0.prj">
  ...

3. Include other patch files in a patch file? E.g.: Will not be implemented.

   <OpenGeoSysProjectDiff>
       <include filename="other_patch.xml">
   ...

---

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behavior change was documented?
4. Does it work with include?