improving the way that existing classes are treated during batch-updates

I've added support for an `action` attribute to all sub-elements of `<classes>` (excluding `<group/>`). It can be set to `"create"` or `"update"`. It can also be absent entirely, in which case it's understood to be `"create"`.

Created classes now always leave all of an item's existing classes untouched. Updated class specs will affect all classes of the named type, replacing the values of all provided class fields, but leaving any unnamed fields untouched.

See the new tests I've added towards the end of `t/controller_Import.t` for some trivial examples.