Feature: per-song normalize

[mpogue2]

Normalize should not be a global option. I have one song (Rhythm's Fireball) that has a high volume spike in it, which causes the whole song to be normalized way too low.

Instead:

• normalization should default to ON (perhaps a preference should specify this, instead of the menu option?)
• a button (perhaps [N] or an icon button with up/down arrows or?) should be available to turn it off for specific song, perhaps right next to the loop controls
• the state of that button should be persistent on a per-song basis

[mpogue2]

I suppose we could also change the normalization to be not by peak, but my audible perceived volume, or something like that. But, that would likely take longer at load time (but it would need to be done just once and the result cached).