[Cell] Experiment with collapsing listeners (where possible) to reduce memory costs of cells

Attempt to minimise the number of instantiated listeners in Cell / IndexedCell / TreeCell / ListCell / TableRow / TableCell / TreeTableRow / TreeTableCell. This is done by using a private internal class to group all listeners into one, rather than use the current approach of one listener per property.

This changeset is experimental and may introduce regressions, so unless the memory reduction is substantial it may not be worth keeping this changeset around (indeed, it is possible this may cost more memory than the previous approach!).