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/reducers/index.js

@@ -4,3 +4,4 @@ export * from './playerReducer'
 export * from './albumView'
 export * from './activityReducer'
 export * from './settingsReducer'
+export * from './replayGainReducer'

ui/src/reducers/replayGainReducer.js

@@ -0,0 +1,45 @@
+import { CHANGE_GAIN, CHANGE_PREAMP } from '../actions'
+
+const getPreAmp = () => {
+  const storage = localStorage.getItem('preamp')
+
+  if (storage === null) {
+    return 0
+  } else {
+    const asFloat = parseFloat(storage)
+    return isNaN(asFloat) ? 0 : asFloat
+  }
+}
+
+const initialState = {
+  gainMode: localStorage.getItem('gainMode') || 'none',
+  preAmp: getPreAmp(),
+}
+
+export const replayGainReducer = (
+  previousState = initialState,
+  { type, payload }
+) => {
+  switch (type) {
+    case CHANGE_GAIN: {
+      localStorage.setItem('gainMode', payload)
+      return {
+        ...previousState,
+        gainMode: payload,
+      }
+    }
+    case CHANGE_PREAMP: {
+      const value = parseFloat(payload)
+      if (isNaN(value)) {
+        return previousState
+      }
+      localStorage.setItem('preAmp', payload)
+      return {
+        ...previousState,
+        preAmp: value,
+      }
+    }
+    default:
+      return previousState
+  }
+}