Request: Secondary track option

[Quackdoc]

iced_video_player currently uses playbin. Playbin allows loading subtitles using suburi= but this actually allows you to load an arbitrary track. Supporting an external secondary track can be necessary to support subtitles, but by supporting suburi you can actually use yt-dlp.

An example pipeline to test this out would simply be;

VIDEO=YT-URL
gst-launch-1.0 playbin uri=$(yt-dlp --format bestvideo --get-url $VIDEO) suburi=$(yt-dlp --format bestaudio --get-url $VIDEO)

I think a good invocation of this could simply be cosmic-player $URI --aux-track $URI2

It may also simply be better to use your own gstreamer pipeline instead of playbin since playbin can actually be rather limiting in controls. It can often be wanted for instance to use swdec and hwdec can often have issues rendering a file, This also won't have as many limitations as playbin only supporting 2 concurrent tracks.