[MovieList.py] Improve and optimize the code

- Sort and optimize the imports.
- Remove the duplicate EXTENSION definitions and use the definitions directly from FileList.py.
- Ignore all JPEG files as they are often added as movie posters but should not be listed as extra media.
- Optimize and PEP8 some of the code.
- Use consistent quotation marks.
- Improve some logging messages.
- Improve some of the comments.

This is not a full refactor of the code though a more comprehensive refactor and optimization will probably be done later.

@jbleyel also contributed to the commit.