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
Makes the styles window a bit higher to list at least 5 styles.
usermanual: proofreading thanks to Rob Z. Smith
lua : make the core-visible lua state a structure

this will allow me to have more lua private data in there later
lua : move the mutex into the new lua structure
add a function to check if we have the gdk lock
lua : run all lua code in secondary threa

lua cannot block the UI thread anymore

note that lua code can still
* starve DT out of working threads
* prevent other LUA threads from working
usermanual: several updates
collect : avoid some gtk-warnings with tags
Updated Dutch translation
Merge pull request #347 from MRIG/master

Updated Dutch translation
Updated Japanese translation
Merge pull request #348 from MRIG/master

Updated Japanese translation
Italian translation updated
usermanual: focus detection example
Fix film import with >1 tags being assigned

All this signal business is quite messy. I hope that I didn't introduce
new bugs. :/
Merge branch 'master' of git://github.com/darktable-org/darktable
Updated Dutch translation