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
Updated French manual (~10% done)
I'm gonna translate the usermanual into French
Updated translation (~10% done)
Makes the styles window a bit higher to list at least 5 styles.
Italian translation updated
Merge pull request #351 from MRIG/master

Updated Dutch translation
collect : correct bugs with items selection
collect : don't show empty "uncategorized" tag entry
collect by color label: new default setting delivers all images having any color label
Merge pull request #349 from bartokk/master

Italian translation updated
Merge pull request #346 from darktable-org/higher-style-window-list

Makes the styles window a bit higher to list at least 5 styles.
Merge pull request #260 from bencahill/master

Improve filter on import
focus: correctly clip for portrait mode images
lua : removed delayed execution

this wasn't used and didn't work
lua : minor cleanups
lua : allow lua callbacks to yield

the argument to yield can be

* "wait_ms",value : yield for the specified number of ms
* "file_readable",file : yield until the file is readable
* "run_command",command : runs the command and yield until the command finishes execution
collect: improve iso and aperture filtering and selection