Ability to download tracks

- Adding the option to download the selected track or tracks.
When the download is clicked it opens a dialog that lets you choose where
you want to save the track and the file name (by default
the file name is the track number, the track name and the extension, example "03 - my song.mp3")
If there are many tracks selected it will open one dialog for each track
- Next steps: download an album and a playlist using zip

backend/mediaprovider/mediaprovider.go

@@ -1,6 +1,9 @@
 package mediaprovider
 
-import "image"
+import (
+	"image"
+	"io"
+)
 
 type AlbumFilter struct {
 	MinYear int
@@ -83,4 +86,6 @@ type MediaProvider interface {
 	DeletePlaylist(id string) error
 
 	Scrobble(trackID string, submission bool) error
+
+	DownloadTrack(trackID string) (io.Reader, error)
 }

backend/mediaprovider/subsonic/subsonicmediaprovider.go

@@ -3,6 +3,7 @@ package subsonic
 import (
 	"errors"
 	"image"
+	"io"
 	"math"
 	"strconv"
 	"sync"
@@ -249,6 +250,10 @@ func (s *subsonicMediaProvider) SetRating(params mediaprovider.RatingFavoritePar
 	return err
 }
 
+func (s *subsonicMediaProvider) DownloadTrack(trackID string) (io.Reader, error) {
+	return s.client.Download(trackID)
+}
+
 func toTrack(ch *subsonic.Child) *mediaprovider.Track {
 	if ch == nil {
 		return nil