ListBox.SelectedIndexCollection modification works but IList versions throw NotSupportedException

Example Problems

E.g.

void IList.Clear()
{
    throw new NotSupportedException(SR.ListBoxSelectedIndexCollectionIsReadOnly);
}

public void Clear()
{
    if (owner != null)
    {
        owner.ClearSelected();
    }
}

Also, the properties IsFixedSize and IsReadOnly return true, but this is quite clearly not the case

Proposed Changes

• Modify IsFixedSize and IsReadOnly to false
• Modify the IList modification methods to call the non-IList methods on the collection: Add, Clear, Remove

Thoughts? This would be breaking, but in the way that is good as we change exception to something functional: see https://github.com/dotnet/runtime/blob/4f9ae42d861fcb4be2fcd5d3d55d5f227d30e723/docs/coding-guidelines/breaking-changes.md

[hughbe]

If we like this, will implement after #3086

[dreddy-work]

Thanks @hughbe for this proposal. Given this has been the behavior for long and could break migrating applications, we like to wait until there is a real customer scenario here.