Added full list of media files (and associated data) from the HEADER section

[JaviMaciasG]

The header of the ELAN eaf files can contain multiple media files, so that I made a simple modification to read them all. It is implemented as a list of dictionaries contanining the data associated to each media file. It follows the implementation of the properties that are already read.

I kept the speach functionality as it was, to avoid possible incompatibilities and to keep things simple. I guess that the setter and getter methods related to the media_file, media_url, mime_type, relative_media_url should be generalized to allow for setting multiple media descriptors, but this can be done later.

[erenyenigul]

This looks perfect! Just what I needed! Please merge this to the main.

[JaviMaciasG]

This looks perfect! Just what I needed! Please merge this to the main.

I don't know if the repo owner is still active as his last commits in GitHub are dated 2022. Let's hope he can find the time to do the merge.