Restructure code for support of Nikon cameras

[capricorn8]

A major part of the code to handle Nikon focus points is not part of the brand specific Delegates file but part of DefaultDelegates, even though it is not used for any other brand and due to the special way how Nikon is treating PDAF info never will. Pentax is similar but has its own specific implementation in PentaxDelegates.

Moving all the Nikon related stuff to NikonDelegates will also help to cleanup this code. E.g. now there are two different places to distinguish between availability CAF and PDAF information for DSLR and Mirrorless, while the general mechanism is the same.