Make SliceProxy respect the collection interface / conventions

This should not change anything, except if you are using the internals of SliceProxy (direct access to _base or calls to _getItem).

As is in the documentation since !129 (merged) (with a small addition in !127 (merged) ), ._base of a collection is the top-level collection, and .idx is the index of an object in that collection. This allowed for some optimisations / hacks (and potentially some generic helpers in the future) - but @fbury noticed that these don't work in some cases, which turns out to be because slices use a different convention (parent as _base and "parent index" where "base index" is expected).

This PR makes SliceProxy follow the same convention (and actually it makes the code simpler :-) ).