cleanup xml mapping names

this is quite a BC break when using xml mappings.

i am not sure if this is perfect yet. i get no more failing tests, but maybe some things just are not covered...

i tried to follow the orm example with the xml attribute names: things mapping phpcr properties or special information like id, locale, nodename and so on use `name` to specify the class field. all relations like parent, child/children, references, referrers use `field` to specify the class field. but we could also argue to use `field` everywhere except on the `<field>` and  `<id>` mappings. the rest of the field-things does not exist like that in the ORM afaik.