Fixed a bug that a preview element was displayed even no preview image was set.
- Adjusted the styles for sortable fields including making all field element float.
- Changed the structure of the filed container elements.
- Made it possible for the user to set attributes of the tags of fieldset, div of fields container, and div of each field container.
- Made it possible for field type definition class to set the position of repeatable field buttons.
- Added an example of setting field attributes in the demo plugin.