Some improvement suggestions

[PrzemoF]

Thank you for making this tool! :-) Some suggestions:

1. Remember input directory
2. Allow saving to a different directory
3. Success could me just a message in the main window. User shouldn't need to make extra click after all went OK. And success window has no OK button.
4. Add info how to run it from the source code. Typical location is in README or INSTALL file.
5. Add installation option (python setup.py)
6. Move from bash script to python. bash is not common everywhere, python is.
7. Use consistent file naming (currently: underscores, dashes, camel style)
8. Use python code quality checker like "black" or "pep8"
9. Move images to a subdirectory with resources - it will look cleaner.
10. Allow selection of files with different extension that pdf. pdf is not always pdf :-)
11. Capture errors from ghostscript. Right now if provided file is not pdf, but has pdf extension user is served with success message even when ghostscript shows error with exit status 1

[abhishekpc]

@PrzemoF Thank you for the suggestions. Some of them are straight forward to implement. Some will take some time.

Hopefully, we'll have most of your suggested improvements in the next release :)