• I have read CONTRIBUTING.md
• This implements/fixes issue Play random files in playlist as a default  #3950.

Description:

Adds explicit support for --shuffle=yes when supplied via the Additional mpv options table in Settings > Advanced.

As noted in this comment:

Fwiw, shuffle: yes WAS working for me, as of IINA for Mac 1.3.4, but it broke again in 1.3.5. :(

Analysis:

1. PR Send directory arg to mpv instead of processing in IINA (fixes #4434) #4439 was the original attempt to add support for --mpv-shuffle arg via iina-cli. It was released in IINA 1.3.4. This also allowed --shuffle=yes via Settings > Advanced to work. But this introduced a regression: issue Opening folder spawns playlist that includes non-media files #4548
2. PR Revert change to folder filtering + add special handling for shuffle arg instead (fixes regression: #4548) #4566 fixed the regression while adding support for --mpv-shuffle arg via iina-cli. This was in IINA 1.3.5. But it unfortunately broke support for --shuffle=yes via Settings > Advanced.
3. This PR extends the previous PR to re-add support for --shuffle=yes using the improved solution.