Picker enhancements

**Added support for multiple component columns in XLFormPickerCell**
XLFormPickerCell will display multiple component columns in the picker view when the row descriptor’s selectorOptions is set to a multidimensional array. The row descriptor’s value will be set to an array of selected values from the components.

It might be a good idea to extract the picker view from XLFormPickerCell into its own view so that it can be used in the XLFormSelectorCell as well.