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_test.go

@@ -41,6 +41,10 @@ var _ = Describe("Tags", func() {
 			Expect(m.FilePath()).To(Equal("tests/fixtures/test.mp3"))
 			Expect(m.Suffix()).To(Equal("mp3"))
 			Expect(m.Size()).To(Equal(int64(51876)))
+			Expect(m.RGAlbumGain()).To(Equal(3.21518))
+			Expect(m.RGAlbumPeak()).To(Equal(0.9125))
+			Expect(m.RGTrackGain()).To(Equal(-1.48))
+			Expect(m.RGTrackPeak()).To(Equal(0.4512))
 
 			m = mds["tests/fixtures/test.ogg"]
 			Expect(err).To(BeNil())