[Dashboard API][Meta] Cleanup dashboard typing

[thomasneirynck]

Some of the API needs tweaking

• object is now nested under item. Remove this nesting. (e.g. meta is SO-specific and should be removed) (perhaps this is needed for the RPC-layer of content-management which is used internally, but it shouldn't be)
• namespaces should just be named spaces
• kibanaSavedObjectMeta should be renamed to something generic (e.g. filterState, ...)
• should we use snake case instead of camel case instead?

[elasticmachine]

Pinging @elastic/kibana-presentation (Team:Presentation)