Do we want to support non-dict backends?

[fwkoch]

This was a thought very early in the life of properties, but nothing has come of it. The idea is that properties may become an interface for a database, where underlying property values are stored somewhere a bit more robust than just a dictionary.

In practice, though, properties has worked well as a user interface separated from the database by serialize/deserialize. The close connection of _backend to a database has not been necessary.

Is there any merit to the idea of "swappable backends"? My current thinking is "no" but maybe there is.