Improve dynamic placeholder implementation

- Rename fields/methods for clarity:
  * dynamic_placeholder_button_hidden -> hide_placeholder_button
  * _compute_dynamic_placeholder_button_hidden -> _compute_hide_placeholder_button
- Restructure JS components:
  * Replace dynamic_placeholder_charfield_patch with new component-based implementation
  * Add proper prop handling and converter type awareness
- Add onchange handler to clear value when converter changes
- Refactor placeholder computation using dictionary mapping
- Update XML views with new field names and component parameters
- Improve mimetype.py documentation and default handling