Clear issues list (again) for 2023.9.0

[fsobolev]

• Closes Option to dowload only subtitles ? #568
  No, the option to download only subtitles wasn't added, but instead now there's no format selection, it's chosen by the app to ensure subtitle will be embedded (SRT for MP4 and VTT for WEBM). The state of "Download Subtitle" switch is now also saved to help users who always download video with subtitles.
• Closes Not all subtitle languages are supported #572
  Two-letter codes with region are now supported. Can be tested with this video and pt-BR language code.
  Also I removed apply button in the entry, it's a bit confusing and doesn't really serve any purpose: we will save configuration on window closing anyway, no need to save it at some other point.
• Closes Keep original channel name in metadata #574
  ffprobe output for the video linked in the issue:

  Input #0, matroska,webm, from 'Ваш топинамбур.webm':
  Metadata:
    title           : Ваш топинамбур
    ARTIST          : Первый образовательный телеканал
    track           : 0
    DATE            : 20210615
    ENCODER         : Lavf60.3.100
  

  You can see ARTIST is now in Russian.
  This is done by finding current culture's name or two-letter code in a hardcoded list of youtube's supported languages. We have to do this, because giving unsupported language code causes yt-dlp to fail, even with ignoreerrors.
• Closes Include URL to video in metadata #576
  A switch was added to preferences:
  
  Off by default. I also noticed there was handler_name field in streams' metadata containing info about Google in case of youtube videos, so this is now cleared too.

[nlogozzo]

The preference should be:
Title: Remove Source Data
Description: Clear metadata fields containing the URL and other identifying information of the media source