TDropDownList displays a dropdown list on a Web page.
It inherits all properties and events from {@link TListControl}.
Since v3.0.3, TDropDownList starts to support optgroup. To specify an option group for
a list item, set a Group attribute with it,
$listitem->Attributes->Group="Group Name";
or in template
Since v3.1.1, TDropDownList starts to support prompt text. That is, a prompt item can be
displayed as the first list item by specifying either {@link setPromptText PromptText} or
{@link setPromptValue PromptValue}, or both. Choosing the prompt item will unselect the TDropDownList.
When a prompt item is set, its index in the list is set to -1. So, the {@link getSelectedIndex SelectedIndex}
property is not affected by a prompt item: the items list will still be zero-based.
The {@link clearSelection clearSelection} method will select the prompt item if existing, otherway the first
available item in the dropdown list will be selected.