Limit usage of legacy CamelObject class.

Only CamelFolder and CamelSession utilize what's left in CamelObject,
which is just binary state files to persistently store a flag or two.

All other Camel classes should inherit directly from GObject.  This
requires a libcamel soname bump since the class structure sizes will
decrease by two pointers (no state_read() and state_write() methods).