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
updated translation
Merge remote-tracking branch 'upstream/master'
Merge branch 'purge-unused-tags' of https://github.com/darktable-org/darktable
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'
Merge remote-tracking branch 'upstream/master'
Order suggestion descending on the actual count.

The order on id does not help at all the user.
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
lighttable: enable full-res thumbnail when pressing `z'.

unfinished, needs better user interface/option in prefs for it.
Merge remote-tracking branch 'upstream/master'
preview: fix memleak and some rounding errors
preview: fall back to old `z' in case there is no thumbnail in the file.