Previously, `Property` records were (incorrectly) just associated with
the library or cell.
This requires a change to how `CellName`s are handled by `OasisLayout`,
since they can have associated properties. They now have their own
non-record object (like `XName`s did before) which holds the properties.
There is also now a `FileModals.property_target` attribute which tracks
which record new `Property` rescords should be associated with.