[tree2] TreeStateBase uses "physical node id"

[Mario Ivankovits]

Hi!

I have seen the tree2 component uses a (I will call it) "physical node 
id" like "0:0:0" to store the expanded/collapsed states.

Now if one inserts a new record into say a database and regenerate the 
tree it might happen that those ids no longer match the data and e.g. a 
"illogical state" exception will be thrown if say 0:1:0 is no longer a 
leaf but a node now. (0:1:0 is 0:2:0 after the above insert)

Shouldnt we use a nodeId provided by the user and thus the user can 
create its own TreeState implemenation to be a little bit smarter when 
trying to determine if a node is expanded/collapsed.

I could have a look at it if we find consens. I think we should bring 
TreeNode.getIdentifier() in game here.

Ciao,
Mario