RAW preview in gwenview

It uses KDcraw to extract the embedded jpeg preview (most of the raw formats
contain it for the purposes of quick viewing the photos on the camera display)
or loads half-scaled preview (otherwise) so it is fast, implementing was quite
easy and theoretically it enables gwenview to support everything dcraw supports
(I haven't found a raw file that gwenview doesn't show with this patch). The
patch does not perform full demosaicing nor any tweaks (set white balance or so)
and I don't think it shall - digikam, darktable or rawtherapee are much more
suited for developing the 'digital negatives'.

The patch enables support for Nikon NEF and NRW formats, Canon's CR2 and CRW,
Pentax PEF, Sony ARW, Fuji RAF, Olympus ORF, Minolta MRW, Panasonic RW2, Samsung
SRW and Adobe DNG. If you want to try another format, just adding its MIME type
to the list in lib/mimetypeutils.cpp shall be enough.

The patch is feature-complete (generating proper thumbnails and viewing the raw
images incl. EXIF metadata) and well usable.

BUG: 158788
REVIEW: 113397