Implement Pythonic properties for XMP metadata

Replace read-only properties + setter methods with proper Pythonic @property/@Setter decorators as requested by maintainer.

Changes:
- Convert all property() declarations to @property/@Setter decorators
- Remove all separate set_* methods (now integrated into property setters)
- Update all tests to use property assignment instead of method calls
- Remove all comments as requested

This makes the API much more Pythonic and intuitive:
  xmp.dc_title = {'x-default': 'Title'}  # Instead of xmp.set_dc_title(...)
  xmp.dc_creator = ['Author']            # Instead of xmp.set_dc_creator(...)

All tests pass and the implementation maintains full backward compatibility.

Author: Arya-A-Nair