Fixed the sorting problem in selection lists with Google Chrome.

1/ Column now has two distinct attributes:
- optionValuesForRender : key/value map for quick lookup
- optionValues: array of { value:, label: } objects
- in order to group values, optionValues can also contain { label:, values: } objects, where "values" is itself an array

2/ These two attributes are built from the values declared in the metadata:
- the XML format has not changed
- the JSON format has changed: became an array as Column.optionValues
- compatibility with old JSON format is ensured through function "_convertOptions"

3/ Regarding custom EnumProvider's:
- function "getOptionValuesForRender" should not change (still expected to return a key/value map)
- function "getOptionValuesForEdit" should now return an array as for Column.optionValues
- compatibility with old EnumProvider's is ensured through function "_convertOptions"

4/ The PHP wrapper has been adapted to generate the new JSON format.
The expected format for $values in PHP has NOT changed (associative array).

5/ SelectCellEditor has been adapted to use the array.
It will thus render values in the expected order, on all browsers.

*CONCLUSION*

Everything stays compatible, but to benefit from a correct sorting on all browsers:
- if you use the PHP wrapper (XML or JSON) nothing to do
- if you use the XML format, nothing to do
- if you use the JSON format, you should adapt the values in "metadata"
- if you use a custom EnumProvider, you should adapt getOptionValuesForEdit