Tree: set aria-setsize and aria-posinset

[bpasero]

The tree/list is virtualized and as such a screen reader cannot make a correct statement on the number of entries. We should leverage aria-setsize and aria-posinset for that purpose within the tree and list widgets. The widgets know the number of elements to render and the currently selected items position in the set.

/cc @joaomoreno @isidorn

[isidorn]

Makes sense, I can look into it in june

[isidorn]

Accidently closed this by PR which only address the list widget, still gotta do it for the tree

[isidorn]

After looking into this with @joaomoreno we have fixed this issue for the list, however for the tree the situation is a bit more complex:

• It is not exactly specified what the aria-posinset and aria-setsize should be with respect to the tree structure. Should they be absolute or only relative to the siblings.
• In both of the cases we have an issue that the tree smartly chooses the minimal set of elements that need to be re-rendered, so if an element A gets removed and element B is above it, B will not get re-rendered and will still have the wrong aria-setsize attribute.

Due to that we would need a bigger refactoring in order to fix this for the tree. So renaming this issue and pushing it to backlog for now.