Camera `state` member is replaced by a `runlevel` which is used at high and low
interface levels and has more possible values (e.g. to dsitingusih between
recoverable and unrecoverable errors).

The semantics of calling the virtual camera methods is better explained and
more consistent.  As a result, the low-level interface is easier to write.
This has been reflected for Phoenix cameras (not yet for Andor ones).