QtIFW

Update usage of Apple-related terminology

Replace deprecated Q_OS_OSX macro used in several places with the newer
Q_OS_MACOS. Old style reference of Apple's desktop operating system name
as "OS X" in documentation and files are also replaced with "macOS"
where applicable.

Task-number: QTIFW-1406
Change-Id: I0561d5e0d964917ac4fca0702d58a06b85d30e57
Reviewed-by: Katja Marttila <katja.marttila@qt.io>