Add notification when a download is completed

- fyne notication looks for the app icon to display it in the notication, so I setted the icon  in `main.go`.
- deleting points after "Download" option in grid view that I mistakenly leave it in a previous PR.
- adding the new paremeter `downloadName`

Screenshot:

Fixes #202

ui/browsing/playlistpage.go

@@ -262,7 +262,7 @@ func NewPlaylistPageHeader(page *PlaylistPage) *PlaylistPageHeader {
 						sharedutil.TracksToIDs(a.page.tracks))
 				}),
 				fyne.NewMenuItem("Download", func() {
-					a.page.contr.ShowDownloadDialog(a.page.tracks)
+					a.page.contr.ShowDownloadDialog(a.page.tracks, a.playlistInfo.Name)
 				}))
 			pop = widget.NewPopUpMenu(menu, fyne.CurrentApp().Driver().CanvasForObject(a))
 		}