nwggrid: rework pins ordering to work with dynamic updates

[Siborgium]

See #176 and 758b19b

The way pins are done now is problematic because it relies on a static & immutable list of pinned entries when determining if freshly appeared entry should be pinned. If entry is deleted & added or changed, it loses its position. The desired behavour should be discussed, but I am pretty sure we don't want changed entries to lose their positions.