Fix volume slider always at max when using custom volume output

[s-aahmedramlan]

Fixes #2899

The volume slider would always reset to maximum when using forced volume above 100%. This fix:

1. Stores user's preferred volume slider position separately
2. Restores the position when loading new videos
3. Applies gain amplification on top of user preference

[ImprovedTube]

Hi and thank you so much! @s-aahmedramlan Sorry for the late reply!

I answered at #2899 Let's not remove/repurose "ImprovedTube.storage.player_loudness_normalization === false", since no-normalization is another feature. (It might not currently work). And the code need not run always