CHIRP

[ft4] driver restructure (#7615)

While implementing two more radios of the Yaesu FT-4 family (the FT-25, see #7543, and the FT-4V, see #7387), I found that with some moderate reorganization, implementation of new radios will become easier. The proposed reorganization implements one more interstitial layer of inheritance to support the sub families of FT-4 (containing the FT-4X and FT-4V) and FT-65 (containing FT-65 and FT-25). Also, some variable assignments have been moved from the individual radio classes to the SCU-35 base class and to the interstitial classes named above.

This change also adds the infrastructure for adding European or Asian models, and as such prepare for addressing a number of currently open issues (6.25kHz tune step issues on EU models, frequency limitations; see #6619, #6651, #6677, #6761, #6869). It will also make it easier for a few additional fixes for issues which were found during my work on this driver (#7601, #7603, #7605). I will submit these fixes in additional patches.

Fixes: #7615