yt-dlc/youtube_dl/postprocessor
Juan M Martínez 374c761e77 [FFmpegPostProcessor] Default of prefer ffmpeg
When no `downloader` is passed to `FFmpegPostProcessor`
an exception was raised trying to get the prefer ffmpeg param.

    AttributeError: 'NoneType' object has no attribute 'params'

This fixes and defaults to `False`.
..
__init__.py Forgot to clean the remains of class
common.py [postprocessor/common] Use 'self._downloader.params' instead of 'self.params'
embedthumbnail.py [postprocessor/embedthumbnail] Skipping if thumbnail file missing
execafterdownload.py ExecAfterDownloadPP: fix __init__ method
ffmpeg.py [FFmpegPostProcessor] Default of prefer ffmpeg
metadatafromtitle.py Postprocessors: use a list for the files that can be deleted
xattrpp.py [xattr] Enhanced error messages on Windows