Add an optional plugin for automatic correction of lens variations based on the lensfun database (and library).
The current status of the lensfun database seems not to be too helpful for most users yet. We need to find a way to help improving this. See the STATUS file for details.

The code in klensfun.h and klensfun.cpp might be used also elsewhere later, for example for automatic correction during picture import.

This plugin gets not compiled, if lensfun library is not installed.