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
- preset data is no longer bundled into iD.js
- some code pathways commented out re: external presets
- many changes so that tests can run without presets at start, or async
- still need to make sure fallbacks are always there (point, line, area, etc)
