Improve filter on import

Instead of showing all images, we set the filter to the smaller of the
default import rating and the current filter. This ensures that the
imported images are shown, but that we don't reduce the filter
unnecessarily.
Update comments for improved import filter handling
Keep old import filter reset for collect operations
A few new strings
lighttable: enable full-res thumbnail when pressing `z'.

unfinished, needs better user interface/option in prefs for it.
preview: fix memleak and some rounding errors
preview: fall back to old `z' in case there is no thumbnail in the file.
Set description in Picasa export only if it is defined

Otherwise Picasa displays filenames as photo descriptions, which is not nice in exported albums
Use Google's terminology in variable names - title/summary, instead of caption/description

Title is the filename and summary is something that is displayed along with the photo on the web (xmp title).
Update Dutch translation
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
update dutch translation
xmp tags: compress bigger binary fields
Merge branch 'master' into zxmp
make XMP tag compression a config option
add italian descriptions to darktable.desktop