tiff output: set sample format for 8bit and 16bit too

DT tiff makes enfuse complain about missing info:
Warning: no TIFFTAG_SAMPLEFORMAT or TIFFTAG_DATATYPE, guessing pixeltype 'UINT16'.

Setting the sample format for 8bit and 16bit cases fixes this.
Add italian keywords in .desktop

Based on PR 471
Add German keywords to .desktop file
Make data/darktable.desktop translatable along with all other things that
need translation.  To translate, translators don't need to do anything
more than they have done so far, it just beomes a few more strings to
translate.
_Keywords got dropped by mistake, restore it
Update French translation.