starting Catalan usermanual translation v1.6 

added 'ca' to doc/usermanual/po/LINGUAS and pulling an initial ca.po for the usermanual (only credits and part of chapter 1 translated by now). It will take some months to me to complete the translation. But I pull request it now to be able to fetch and merge updates in my working directory in the meantime.

I have not added nor commited the changes in my local 'darktable-usermanual.pot' file, I am not sure what I should do with it, it contains local links to other files in my hard disk.