Usermanual: mention slideshow view and overwrite on export
Add dt_ prefix to non-static histogram fucntions
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