This commit introduces a new `filteredItem` field called `index` which
stores the index of the filtered item in the unfiltered list. This
allows to get at runtime the unfiltered list index for the selected (and
possibly filtered) item.
This is the only solution I found to use `SetItem` with a filtered
list.
The name `GlobalIndex` might not be ideal, I'm happy to change it to
something else. (`UnfilteredIndex`?)
Fixes: #550
