Improve Collapsed ExpandItems display

[philnate]

Currently the ExpandItem shows only the _id of a document if available (except if it's system.indexes see commit 66848fc). In many collections this _id may be of no use as more complex primary keys, which are used through the app are used and/or other data is more important. So I think it would be cool if for each view it could be defined what to display as ExpandItem summary.

So per default all collections would come with the primary _id summary, except system.indexes which comes right out of the box with something more meaningful.