IntervalStore should preserve order of addition of co-located features

Add features f1, f2, f3, f4 (of the same type) at the same position of a sequence.
Tooltip lists them in reverse order f4, f3, f2, f1.
Exporting the features, and reloading them, reverses the order again. This can result in non-reproducible visualisation of an alignment.
Co-located features should be reproducibly stored in the order of addition.