ExifTool wrapper : introduce applyChanges() method to update existing tags or to add new tags.

The method take a TagsMap hash-table to host ExifTool tag names as key and tag values.
Typically, the key follow ExifTool CLI tool argument typo as EXIF:ImageDescription. The value is a UTF8 string.
This method is not yet used, unit tests must be written in first.
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