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
highlights: be less aggressive.

works fine for my single test shot. more testing would be great (+opencl version pending)
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'
Add helper function to get a filesize
Don't try to import empty file

See bug #9578.
Bug #9578: Don't import dot folders

Dot files are still imported though ...
Fix #9578: This time also for dot files
Don't write metadata on startup. Stupid bug ...
usermanual: a bit of this and that