Recode value order - #478

Values of the value classifications displayed in the simple style editor where sorted according to the string lexicographic order, even for numeric values. This PR tries to change that by offering the ability to give a dedicated Comparator to a MappedLegend. We use this feature to have a dedicated order when dealing with AbstractRecodedLegend whose classification is made on a numeric field. 

Such a Comparator is given: 
- when creating classifications in the editor 
- when computing the classification in order to open the editor so that the display is good on opening.