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

ui/widgets/tracklist.go

@@ -70,12 +70,13 @@ type Tracklist struct {
 	DisableSorting bool
 
 	// user action callbacks
-	OnPlayTrackAt   func(int)
-	OnPlaySelection func(tracks []*mediaprovider.Track, shuffle bool)
-	OnAddToQueue    func(trackIDs []*mediaprovider.Track)
-	OnAddToPlaylist func(trackIDs []string)
-	OnSetFavorite   func(trackIDs []string, fav bool)
-	OnSetRating     func(trackIDs []string, rating int)
+	OnPlayTrackAt        func(int)
+	OnPlaySelection      func(tracks []*mediaprovider.Track, shuffle bool)
+	OnAddToQueue         func(trackIDs []*mediaprovider.Track)
+	OnAddToPlaylist      func(trackIDs []string)
+	OnSetFavorite        func(trackIDs []string, fav bool)
+	OnSetRating          func(trackIDs []string, rating int)
+	OnShowDownloadDialog func(track *mediaprovider.Track)
 
 	OnShowArtistPage func(artistID string)
 	OnShowAlbumPage  func(albumID string)
@@ -508,6 +509,10 @@ func (t *Tracklist) onShowContextMenu(e *fyne.PointEvent, trackIdx int) {
 					t.OnAddToPlaylist(t.SelectedTrackIDs())
 				}
 			}))
+		t.ctxMenu.Items = append(t.ctxMenu.Items,
+			fyne.NewMenuItem("Download", func() {
+				t.onDownload(t.selectedTracks())
+			}))
 		t.ctxMenu.Items = append(t.ctxMenu.Items, fyne.NewMenuItemSeparator())
 		t.ctxMenu.Items = append(t.ctxMenu.Items,
 			fyne.NewMenuItem("Set favorite", func() {
@@ -582,6 +587,12 @@ func (t *Tracklist) onAlbumTapped(albumID string) {
 	}
 }
 
+func (t *Tracklist) onDownload(tracks []*mediaprovider.Track) {
+	for _, track := range tracks {
+		t.OnShowDownloadDialog(track)
+	}
+}
+
 func (t *Tracklist) findTrackByID(id string) *mediaprovider.Track {
 	idx := sharedutil.Find(t.tracks, func(tr *trackModel) bool {
 		return tr.track.ID == id