This way, the a11y code knows if a column was reordered, added or
removed and can do the right things instead of trying to guess and
getting it wrong.
Also, this patch finalizes the changes so that only visible columns
exist to the accessibility interface.
Track the RBNode/RBTree instead of keeping a TreeRowReference. This is a
whole lot faster and less error-prone.
Also, notify the accessible of removal of rows before actually removing
them, so we have a chance to clean up.
- Name it properly
- Make it just a set of cell_infos
Currently it's using direct hash, but as long as we don't actually
lookup stuff from it, who cares...
This way, we can include them without accidentally including deprecated
code. Which means we can still use the recently added turning-off tricks
for deprecation warnings.
