feat: List "items" method should return instead of setting items

[nervo]

The same way tree has a Child method to set child(ren) and a Children method to return children, list method Items should return items.

Please, take into account that:

• without this, there is currently no other methods to get list items
• list Itemans Items methods are currently redundants, as they both set list item(s)
• this is a breaking change, but list component is not officially released

Fix #320