SelectOptGroup(label: String, options: List<StringPair>? = null, maxOptions: Int? = null, disabled: Boolean = false, classes: Set<String> = setOf())
label - the label of the group
options - an optional list of options (label to value pairs) for the group
maxOptions - maximal number of selected options in the group
disabled - renders a disabled group
classes - a set of CSS class names
Constructor