feat: an option to not use the global "sounded speed" setting at all

[WofWca]

After 1c82c22 I thought that for some people it would be better if we did not attempt to change the default playback rate of media elements at all. For example, on one website they watch videos at 2.5x speed, on another it's 1.5x, but they want silence skipping to work on both of them. But currently our extension always sets sounded speed to the value stored in its settings, so if they visited the first website (where they watch at 2.5x), and then went to the second one (where they prefer to watch at 1.5x) – tough luck – our extension will set it to 2.5x.

I guess we could make the "sounded speed" setting not global but per-element. Also, maybe this should apply to "silence speed" as well? And other settings?

Also, how about instead of having such setting stored somewhere in the extension's code, we simply use el.defaultPlaybackRate is the value for it?

For this we can look up how the Video Speed Controller extension works.