FileDisplayer - pgup/pgdown jumped on wrong line

1. pgup - jumped 2 lines below the correct line (incorrect +-1)

2. pgup/pdgown with combination of number should jump to
     (rows - 1) * number

     but was

     rows * number - 1
Merge pull request #77 from rbalent/file-displayer-pgup-pgdown

FileDisplayer - pgup/pgdown jumped on wrong line