improve table linking

[HelenaLC]

• We currently have a few helpers to help keep coherent links between tables & shapes/labels; in utils.R:
  • .sync_tables_sdattrs updates table spatialdata_attrs
  • .sync_shapes_on_drop filters observations in linked shapes
  • .sync_tables_on_drop filters tables when dropping shapes/labels
• A design draft to keep all links as object-wide .zattrs is here
• Since this could take a while, might be worth to see how we can improve & test more the current R implementation; it is quite in-elegant and confusing...