Move app-global constants into pch.
Move NNAXAPIDisabledNotification into constants.
Replace all calls to AXAPIEnabled with a wrapper.
Different mechanisms for requesting AX access based on Carbon API availability.

Fixes #17
Use the right API based on OS and leave breadcrumbs for future cleanup.
Factor the disabled window controller so it's only created when needed.
Merge branch 'numist/AXAPI-ng' into develop