Move ParseDate onto a serial dispatch queue; perform async initialization after the app finishes launching.

On my Mac mini with SSD and 10.10.1, these changes cut launch time from ~0.9s to 0.4s.  It’s pretty hard to make it all the way to editing the date within 0.5s of launch, but it improves subjective responsiveness quite a bit.

Do a better job of preserving time/date fields with changes to the natural language parser, just in case startup takes forever.  (Previously if one of these were focused, editing would just be aborted; now we still do that, but try to recreate your place afterward.)

Clean up locale/time zone change notification handling so they don’t have to go through PSTimeDateEditor.  This does now unnecessarily send -[PSTimeDateEditor _update] on time zone changes, but it simplifies things and creates fewer/less confusing code paths.
Fix code style.
Pester to do.oo3: Updated status.