Implements tree-state checkbox functionality.
| checkAll | check all items in tree |
| checkItem | checks the checkbox it the tree item |
| getChecked | returns ids of the checked items |
| isChecked | checks whether the specified node is checked |
| uncheckAll | uncheck all items in the tree |
| uncheckItem | unchecks the checkbox in the tree item |
| onItemCheck | fires when you check an item in tree and treetable |
| threeState | defines three-state checkboxes for the tree |