Give a change to ASTableView's delegate to configure each node's container cell before it is shown.

In the current version, instances of _ASTableViewCell are reused / created with default values for all of its properties. They have a white background, default selectionstyle and many others.

Exposing the tableviewcell in the delegate method triggered just before the node is shown allows ASTableView's delegate to configure the container cell (background color, selection style, etc...)