1)GUI changes
 - toolbox removed, source/message tools on one page
 - combo box updated
 - checkbox for separation mark of coded message
 - horizontal line
 - LCD like display for entropy and coding efficiency indicator
 - separate menu both for for source and message file manipulations
2)API
 - entropy and coding efficiency calculation
 - presentation of probability switched from number of appearances
   to probability
TODO-top priority:
 - top-bottom source building method implementation
 - symbol sequence input dialog box
 - prepare documentation
TODO:
 - automatic resizing fit of source tree visualization
 - symbol grouping