plug-ins: Add support for importing CMYK PAM files

Alx Sarequested to merge
alxsa-cmyk-pam-import into master

While not an "official" tupl type for PAM files, software such as mutools (look at "-c colorspace" option) can export CMYK PAM files. This patch adds support for loading such images.

Note that PAM files do not include color profile information. In the future, we might add an option to convert using a pre-defined color profile for this and other formats like EPS.

Sample files:

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