Allow to add extra EXIF tags to images

[giacomomarchioro]

Hello and thanks for this wonderful software.

We would like to add extra EXIF tags such as XResolution, YResolution and ResolutionUnit which are valid EXIF tags not present in RawTherapee 5.8 and hardcoded in 5.9. We tried to add them directly to the .pp3 file, they are seen by RT but they are not saved on the final file.

At the moment we are doing some post-processing using exiftool:

exiftool -Xresolution=600 -Yresolution=600 -ResolutionUnit=inches myimage.tif

But it would be great to avoid this extra step and this tag could be used for implementing other issues: https://github.com/Beep6581/RawTherapee/issues/4894