Add a data area to the Value interface and ValueType implementation
For certain Exif tags the regular value (in the IFD) is just an offset to a data area outside of the IFD. Examples are Exif tag 0x8769 in IFD0 (points to the Exif IFD) or tag 0x0201 in IFD1 (JPEG thumbnail image).
A concept is needed to allow linking this data area to a value, if desired. Ifd will need to be aware of the data area.
The new concept will be used to support the creation of Exif thumbnails from scratch.