Allow for configurable treeColumn property on TreeTableView

Sometimes it is desirable to not have the indentation and disclosure node shown on the left-most column. For example, refer to the attached screenshot. To support this we should allow for the user to specify a treeColumn in TreeTableView. By default the value will be null to indicate to use the left-most column. If it is non-null and the column is a visible leaf column, the indentation and disclosure node will instead be shown in that column.