add support for 'optgroup' on selects

[geopossachs]

is a model based on SelectItemGroup and SelectItem, we could render:

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/optgroup The HTML <optgroup> element creates a grouping of options within a <select> element.

https://docs.oracle.com/javaee/6/api/javax/faces/model/SelectItem.html https://docs.oracle.com/javaee/6/api/javax/faces/model/SelectItemGroup.html