Fixed crop area size

Hi. It would be great to implement this functional:
1. Admin can limit max size of crop area and user will be able to choose any size between min and max sizes.
2. Admin can set "fixed" crop area. It means that user has no possibility to modify crop area size (disable creation of new selections and disallow to resize crop area).
Here it is patch which adds some additional settings in field widget settings form and modify field widget initialization.