Fix incorrect UTI registrations

[jgvanwyk]

This is a continuation of #1468. This pull request fixes the incorrect UTI exported/imported UTIs as well as updates CFBundleDocumentTypes to use LSItemContentTypes instead of CFBundleTypeExtensions.

(This is a new pull request because I screwed up the previous repository while trying to synchronise it with the main branch).

[jgvanwyk]

One thing I noticed while updating the CFBundleDocumentTypes was the use of LSIsAppleDefaultForType, which appears to be undocumented (and I would guess is intended to be private). Should this be removed?

[eirnym]

I think it's ok if it's widely used.