Adjust logic to visualize Nikon DSLR focus points

[capricorn8]

The current (and longstanding) implementation of the plugin does not always show the proper focus point for an image captured with a Nikon DSLR. In this context, "proper" means the focus point position as displayed by Nikon's applications Capture NX-D and NX Studio (and most likely, to the camera display as well).

The underlying problem arises from a combination of the following facts:

• the way how Nikon stores different pieces of focus point related information in EXIF
• possibly, changing this over time and across different models
• not enough background information on the exact meaning of different entries while decoding them for exiftool
  (PrimaryAFPoint, AFPointsUsed, AFPointsInFocus, AFPointsSelected)

See also https://exiftool.org/forum/index.php?topic=17011

The logic should be adjusted to

• match the focus point display of Nikon
• be able to respond to model specific peculiarities (due to inconsistent EXIF decoding by exiftool).