Modernize and unify filesystem code on Apple platforms.

Replace deprecated File Manager APIs with modern equivalents.

Change some Q_OS_MACX to Q_OS_DARWIN in file system related code.

All of these apply to iOS as well as OS X, and were ifdef'ed for OS X
only primarily due to legacy reasons - carryovers from Qt 4 or Carbon
APIs which have since been refactored into using CoreFoundation.

This also makes the code consistent with the documentation.

Change-Id: I414e9bdfffff731413ddf16171b1317027d87caf
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@theqtcompany.com>