use filename instead of title for generate_keyframes_from_scenes

The name of the file the video is saved to is already cleaned. However, the title might contain characters that are not allowed for folder/file names (such as '/', '.', etc.). Therefore, the path should be made from the name of the video file, instead of the actual video title, as is done with the scene extraction.

As an example the current video downloaded in main.py will trigger an error, as the title contains a '.' at the end.