Persist volume level between page loads

[sabjorn]

Based on a 2 star review:

Thank you, but the extension doesn't preserve the volume value - refreshed page results in volume reset.
Developer, please check some other extensions which implemented volume value storage, and it works.

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This will be easy enough add. We can store the volume level in the defaultConfig struct from config_backend.js and then pass this data to/from player.js.