Instantiate this interface via new-item.
| (send a-hierarchical-list-item get-editor) → (is-a?/c text%) | 
Returns a text-editor buffer whose content is the display
representation of the item. In other words, fill in this text editor
to set the item’s label.
| (send a-hierarchical-list-item is-selected?) → boolean? | 
Reports whether the item is selected.
| (send a-hierarchical-list-item select on?) → void? | 
| on? : any/c | 
| (send a-hierarchical-list-item click-select on?) → void? | 
| on? : any/c | 
Calls select or click-select. The on? argument can be #f only if
allow-deselect in hierarchical-list% allows it.
| (send a-hierarchical-list-item user-data) → any/c | 
| (send a-hierarchical-list-item user-data data) → void? | 
| data : any/c | 
Gets/sets arbitrary data associated with the item.
| (send a-hierarchical-list-item get-clickable-snip) | 
| → (is-a?/c snip%) | 
Returns the snip that (when clicked) selects this element the
list. This method is intended for use with an automatic test suite.
| (send a-hierarchical-list-item get-allow-selection?) | 
| → boolean? | 
| (send a-hierarchical-list-item set-allow-selection allow?) | 
| → void? | 
| allow? : any/c | 
Gets/sets whether this item is allowed to be selected.