Tree and ItemList controls should operate on objects, not indices

[blackears]

I've tried working with ItemList in the past and am now trying to implement a Tree, and finding Godot's implementation of both of these controls very hard to work with. In particular, the control is completely separate from the data it is trying to model and you have to synchronize the two by counting index numbers. After working with similar structures in Qt and Java Swing, I think it would be very useful to replace these controls with something that works more like they do.

In particular, it should be possible to give the base object some sort of a model object and then have the tree or list automatically generate itself by querying the model. The model could also emit events to indicate that items have been added or removed. You can customize how items are displayed by passing you custom component as a PackedScene to the tree or list control and creates instances of it for each line item.