Add native Darwin handler for the HEIF (.heic) format

Adds basic support for still images encoded with the HEIF standard
introduced in macOS Sierra and IOS 11.

Uses the Core Graphics Image I/O Framework as codec backend.
Reuses the helper class from the macjp2 handler.

Change-Id: I5f0c7891b189a916cccd2c27eacbac12416ce209
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>