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.
Doc spelling cleanup
unity: don't update launcher progress on wrong job type
wb_presets: update and expand Nikon D7100 (thanks to Chris Siebenmann)
(cherry picked from commit 25a049a0cd889e636e1161a4fa6026f4ffd54f18)
reenable hinter msg for drawn masks
Italian translation updated
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).
rawspeed: import svn r596 (for Canon G16)
adobe_coeff: Canon PowerShot G16 matrix
Merge pull request #314 from bartokk/master

Italian translation updated
lua : autodocumentation, initial implementation
prevent crash when exporting exactly 9 images

Apparently our fraction calculations yeld results >1.0 for some specific number of images
Allow valid UTF-8 sequences in the metadata editor

The offending code was added in response to bug #8744. GTK is documented to
expect valid UTF-8 sequences as input, and is free to crash on invalid input
otherwise. The original fix, however, simply replaced non-ASCII characters with
dots which is clearly not accept able for someone named like me. This change
instead skips this replacement when the string is a valid UTF-8 sequence.

The g_utf8_validate must be invoked without the size being passed. If we pass
around the expected size, then this function fails because it hits the first
NUL byte, a terminator, prior to hitting the length limit, and this is
documented to return false.