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

assigned to @vincent.hatakeyama

enabled an automatic merge when all merge checks for 8898d80c29fe pass

unassigned @vincent.hatakeyama

merged