ReplayGain support + audio normalization (web player) (#1988)

* ReplayGain support

- extract ReplayGain tags from files, expose via native api
- use metadata to normalize audio in web player

* make pre-push happy

* remove unnecessary prints

* remove another unnecessary print

* add tooltips, see metadata

* address comments, use settings instead

* remove console.log

* use better language for gain modes

ui/src/actions/index.js

@@ -2,5 +2,6 @@ export * from './player'
 export * from './themes'
 export * from './albumView'
 export * from './dialogs'
+export * from './replayGain'
 export * from './serverEvents'
 export * from './settings'

ui/src/actions/replayGain.js

@@ -0,0 +1,12 @@
+export const CHANGE_GAIN = 'CHANGE_GAIN'
+export const CHANGE_PREAMP = 'CHANGE_PREAMP'
+
+export const changeGain = (gain) => ({
+  type: CHANGE_GAIN,
+  payload: gain,
+})
+
+export const changePreamp = (preamp) => ({
+  type: CHANGE_PREAMP,
+  payload: preamp,
+})