update VU meter and peak controls

[ronso0]

Basically the update is PascalCase to snake_case, eg. PeakIndicator -> peak_indicator

[SamWhited]

I'm also seeing:

warning [Controller] ControlObject accessed via deprecated key "[Master]" "VuMeter" - use "[Main]" "vu_meter" instead

Suggesting that [Master] has been renamed in addition to the snake_case change.

EDIT: oddly though, it appears that if I try to use [Master],vu_meter it doesn't work, but if I use [ChannelN],vu_meter it works fine, and [Main] doesn't work anywhere, so the Mixxx error may also be wrong.

EDIT 2: found the related issue (mixxxdj/mixxx#14571), looks like this is already known. Sorry for the noise.