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)
- Many text expects are now wrapped in setTimeout, as the fetch
promises settle async now.
- This makes the tests somewhat brittle, and we should maybe consider
reworking some of them. For example it is very hard to perform
a test like `expect(spy).to.have.not.been.called` in an async way.
(We could instead inspect the fakeServer requests() to know this.)
- Also includes some trickery for osm.js, which uses d3-xml (fetch)
now for unauthenticated calls and osmauth (xhr) for authenticated calls
