Registry object search widget:
  - removed stale demo file
  - fixed a bug where subsequent views of the modal wouldn't honour predefined
    'class' and 'datasource' settings
Registry object search widget:
  - added a 'lock_presets' configuration option (default false), which locks
    values for 'class' and 'datasource' if they were supplied.
  - updated demonstrator documentation to reflect new option.