Dialog classes moved into one file - dialogs.py

    Other changes:
- Previous files with dialog classes deleted
- Dialog classes now in the dialogs.py
- Little bit resized wizard window
- Added some comments to wizard class
- Bug fixed that Dialog and FileDialog objects wasn't creating (added imports)
- Instead of button "Next" on last page now there is "Finish" button
- Remade "Changelog" dialog - looks more user-friendly
- Added subtitles to wizard's pages
- Deleted unnecessary attributes on pages (labels)