Fix a case where values wouldn't be requested properly.

- If a node was already expanded, and we then removed/replaced
  its children, those wouldn't automatically get their value nodes
  added. Consequently, value retrieval for them would fail until
  the parent was collapsed/re-expanded. If we encounter such a
  model node when receiving a value request, notify the table model so
  it can construct the value node appropriately.
BOpenHashTable::Clear(): Set fItemCount to 0

If not empty, the count would afterwards be out of sync with reality.