This is construct only property which allows us to do more complicated testing.
This also caches lookups based on Purple.Account:protocol-id.

Opened in a devenv and verified my accounts worked.
Started creating a new account and verified that changing the protocol didn't throw any warnings.
Modified an existing account by changing its protocol without issue.
Deprecated purple_account_get_protocol_name as there's no good reason for it.
Called in the turtles.