US24: Add properties to the DngWriter

- Add properties to the DngWriter and dialog to set the preview size and index image file name.
Start a fix for the saturation problem.
Add new user story
Merge branch 'master' into develop
Merge branch 'fixsat' into fix1

Conflicts:
	Image.cpp
FIX#1: Fix saturation levels and alinearitites.

- Fix the calculation of the saturation level. Use the minimum among the maximum
value of each channel and the saturation value given by LibRaw.
- Fix alinearity problems with some cameras, setting a threshold on the highest values.
Remove some unfinished features from master.