Allow absence of video file in the project file

The presence of the video file in the ZIP project file should be optional. If it is absent, it must be present in the same directory as the project file, with the name indicated in the video field of the metadata.yaml file. Otherwise, a mechanism for indicating the path of the URL (in a cross-system compatible way) should be implemented. Also, a field containing the SHA-1 of the video file should be added to the metadata.yaml file, such that the consistency can be checked on the fly.

Tasks:

• Save and load file size and file SHA-1 digest in metadata.yml
• If the video file is absent from the project file, search for it in the directory where the project file is
• Check for size and SHA-1 digest, in any case (absence from or presence in project file)
• Change to format v4; document this in FORMAT.md
• Write Files.update_metadata method
• Add a menu entry for including/excluding the video file from/in the ZIP file at the next saving