Provides a form element for a drop-down menu or scrolling selection box.
Properties:
#options: An associative array, where the keys are the values for each option, and the values are the option labels to be shown in the drop-down list. If a value is an array, it will be rendered similarly, but as an optgroup. The key of the sub-array will be used as the label for the optgroup. Nesting optgroups is not allowed.
#empty_option: The label that will be displayed to denote no selection.
#empty_val