Experimental LensFun support added into PhotoFlow

An initial implementation of LensFun support has been added into PhotoFlow.

The tool is still experimental, and limited in functionality:

• the camera and lens information can only be extracted from the EXIF metadata, and not specified by hand
• only distorsion and TCA corrections are implemented for the moment, the vignetting is still missing
• there is no possibility yet to select the corrections to be applied
• the interpolation is performed using a simple nearest neighbour method, to be replaced with a better interpolator in future

Nevertheless, if you are interested in this feature and you want to help me in the testing and debugging process, the tool can be accessed from the "Geometry" tab of the tool selector. The tool is called "Optical corrections":

Here is how the tool dialog looks like:

The LensFun tool is for the moment only available from the GitHub sources.
Two additional dependencies are needed to compile PhotoFlow: exiv2 and LensFun.

As any other experimental code, it still contains bugs and is likely to produce frequent crashes, therefore I recommend to frequently save your work while testing it.