Usermanual: mention slideshow view and overwrite on export
usermanual: minor updates and additions
Merge pull request #524 from LebedevRI/usermanual

Usermanual: mention slideshow view and overwrite on export
usermanual: cleanup
usermanual: update on zone system module
usermanual: a few words on number of threads on a 64-bit system
usermanual: a few words on thumbnails and skulls
Deflicker: mode switching: gui complete
DEFLICKER_MODE_SOURCEFILE is working?
Deflicker "Automatic" is working?
Speedup DEFLICKER_HISTOGRAM_SOURCE_SOURCEFILE by caching histogram