[txt backend] ceiling pages value, (0.2 > 1).

This will fix opening small docs, that doesn't have one full page (for
now 6000 symbols)
And also fix last page for huge documents too.
[txt backend] error messages refactoring, surround by #ifdef TXT_DEBUG.
[txt backend] fully rewrite using Okular::TextDocumentGenerator

- Number of code lines was reduced significantly
- Code become clear