do no longer autoapply dithering; modules needs to be activated by user

this avoids that images are marked as modified even if history stack
is purged
do nothing in dt_dev_read_history() if no modules are loaded

this avoids warnings like the following if history stack is discarded from within lighttable mode:
[dev_read_history] the module `sharpen' requested by image `IMG_7273.CR2' is not installed on this computer!
usermanual: small fix to allow inlinegraphics in levels doc
opencl: make sure histogram is shown in tonecurve/levels module even if  preview runs on OpenCL
noise profile: another sony nex-5n, numbers are spot on :)
Minor code reorganisation.
Code refactoring to expose dt_ratings_apply_to_image.
Add support for importing rating from Lightroom XMP.
Add support for importing geo-tagging from Lightroom XMP.