bounds / rect; updates

IIUC the bounds of items should always be specified. Is this true?

#82 suggests that it may be true, though given that bounds were not added there presumably there are other uses (I also see that character_positions assumes some data from bounds).

And yet, I see the representation of Node considers bounds an optional property.

When resizing a GUI most Node properties won't change. From what I can tell, the only reported data affected is bounds and character_positions (not applicable to everything).

So it might possibly make sense to represent this information separately, or to support updating bounds only (e.g. add a field like Vec<(NodeId, Rect)> to TreeUpdate).

The main motivation I see here is actually that it would allow widgets to emit position updates without caring about whether or not they are present in the accessibility tree: accesskit could easily filter out any updates which don't apply to nodes it knows about.

Start with the Node representation and TreeUpdate, then inspect how bounds and character_positions are consumed, including the behavior discussed in #82. Done means establishing whether bounds are always required and choosing a concrete approach for bounds-only updates that preserves correct accessibility-tree behavior.