The file Metadata manipulation Qt wrapper:

- containers : all metadata classes to group information by categories.
- engine     : the low level metadata extraction class based on Exiv2 API and managing :
                   * C++ exception from this library.
                   * All API call protected by a common mutex to fix non re-entrancy from Exiv2.
                   * All Exiv2 API are only used in this area to not expose all digiKam core classes of API changes.
- dmetadata  : the high level metadata class, based on metadata engine, but not based on Exiv2 API.
               This class must be used everywhere in digiKam.
- exiftool   : classes to wrap exiftool command line tool.
- focuspoint : classes,to hanlde focus point information from camera.

NOTE: with Exiv2 less than 0.28, C++11 must be used instead C++17. A definition EXIV2_AS_CONST is
set to constify containers to loop with for() operator.