We don't pay attention to item-type anymore, so
drop the item-type property and the _for_item_type()
constructor, and allow passing NULL to the regular
constructor.
We don't make this constructor transfer-full, since
the selection filter model is not a wrapping model
like the others. It is more like fork than a wrap.
When the position is 0, we can't check for unchanged
elements below with gtk_bitset_size_in_range. And
we don't need to, either.
And be careful when translating [start,length]
intervals to [first,last] ones. Off-by-one errors
lurk everywhere.
