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

scanner/metadata/metadata.go

@@ -143,6 +143,39 @@ func (t Tags) Size() int64                 { return t.fileInfo.Size() }
 func (t Tags) FilePath() string            { return t.filePath }
 func (t Tags) Suffix() string              { return strings.ToLower(strings.TrimPrefix(path.Ext(t.filePath), ".")) }
 
+// Replaygain Properties
+func (t Tags) RGAlbumGain() float64 { return t.getGainValue("replaygain_album_gain") }
+func (t Tags) RGAlbumPeak() float64 { return t.getPeakValue("replaygain_album_peak") }
+func (t Tags) RGTrackGain() float64 { return t.getGainValue("replaygain_track_gain") }
+func (t Tags) RGTrackPeak() float64 { return t.getPeakValue("replaygain_track_peak") }
+
+func (t Tags) getGainValue(tagName string) float64 {
+	// Gain is in the form [-]a.bb dB
+	var tag = t.getFirstTagValue(tagName)
+
+	if tag == "" {
+		return 0
+	}
+
+	tag = strings.TrimSpace(strings.Replace(tag, "dB", "", 1))
+
+	var value, err = strconv.ParseFloat(tag, 64)
+	if err != nil {
+		return 0
+	}
+	return value
+}
+
+func (t Tags) getPeakValue(tagName string) float64 {
+	var tag = t.getFirstTagValue(tagName)
+	var value, err = strconv.ParseFloat(tag, 64)
+	if err != nil {
+		// A default of 1 for peak value resulds in no changes
+		return 1
+	}
+	return value
+}
+
 func (t Tags) getTags(tagNames ...string) []string {
 	for _, tag := range tagNames {
 		if v, ok := t.tags[tag]; ok {