[Nodes] When addind nodes to children, the event fired on the parent listener contains a wrong prev_snapshot

[AxelVoitier]

Found in testing of ChildrenArray. When adding nodes with the children attached to a parent_node, the event it fires on the parent node listener for the addition of child nodes contains a wrong prev_snapshot. It is basically just a mirror of snapshot.

Eg. when adding one node:

event.snapshot=(
    MinimalNode(system_name=None, display_name=node1),
    MinimalNode(system_name=None, display_name=node2),
)
event.delta=[MinimalNode(system_name=None, display_name=node2)]
event.delta_indices=[1]
event.prev_snapshot=(
    MinimalNode(system_name=None, display_name=node1),
    MinimalNode(system_name=None, display_name=node2),
)

[AxelVoitier]

So, prev_snapshot == snapshot because no from_ was given to the NodeMemberEvent.

No from_ is given because, according to the documentation of Node._fire_sub_node_change, nodes_from is only for when nodes are removed, as this relate to where the indices should be taken from.

But then the problem is more about NodeMemberEvent.prev_snapshot to be a misnomer... Because this is actually only used for when nodes are removed. The fact that if it has no previous snapshot recorded it instead returns the current snapshot is very confusing...
Point in case: If you do provide an actual previous snapshot when adding nodes, then the own NodeMemberEvent.delta_indices property gets all confused and raise an exception...

This leave the impression it does this to just read from one snapshot source, instead of doing a simple if self.__add ...

And maybe prev_snapshot should either return None in case of add, or always provide a real previous snapshot, and then delta_indices gets fixed for handling the case of using different source of snapshot depending on if it is a add or a remove.