this necessary when a regional preset (e.g. from NSI) is supposed to inherit fields from a parent preset, but the direct parent does NOT apply at the location of the entity to be added/edited. In that case we need to search for a potential regional variant of the parent preset.
Consolidate localization behavior and init to a coreLocalizer function and singleton
Explicitly support `en-US` locale
Rename coreData to coreFileFetcher and export a singleton rather than using a property of coreContext
Add `apiConnections` property of coreContext to simplify adding a source switcher
Replace some init functions with re-callable, promise-supporting `ensureLoaded` functions
Make coreContext itself load the UI if a container has been specified at init time
Fix code tests
These changes are needed now that `addNode`
* wants to preserve circularity
* automatically remove duplicates
* range checks index argument (can't call it with -1 anymore)
