Now it actually defaults to "a < b" comparison, instead of forcing you
to provide a trivial less-than comparator. Also you can pass in any
collection type that has .begin() and .end() and we'll sort it for you.
There is some sort of issue with using a SortingProxyModel together
with ColumnsView. This is a workaround to allow FilePicker to use a
MultiView for now, but this needs to be fixed separately somehow.