add select all shortcut to album and playlist pages

ui/browsing/albumpage.go

@@ -108,6 +108,10 @@ func (a *AlbumPage) Tapped(*fyne.PointEvent) {
 	a.tracklist.UnselectAll()
 }
 
+func (a *AlbumPage) SelectAll() {
+	a.tracklist.SelectAll()
+}
+
 func (a *AlbumPage) onPlayTrackAt(tracknum int) {
 	a.pm.PlayAlbum(a.albumID, tracknum)
 }

ui/browsing/browsingpane.go

@@ -31,6 +31,10 @@ type Searchable interface {
 	SearchWidget() fyne.Focusable
 }
 
+type CanSelectAll interface {
+	SelectAll()
+}
+
 type CanShowNowPlaying interface {
 	OnSongChange(song *subsonic.Child)
 }
@@ -89,6 +93,12 @@ func (b *BrowsingPane) GetSearchBarIfAny() fyne.Focusable {
 	return nil
 }
 
+func (b *BrowsingPane) SelectAll() {
+	if s, ok := b.curPage.(CanSelectAll); ok {
+		s.SelectAll()
+	}
+}
+
 func (b *BrowsingPane) doSetPage(p Page) bool {
 	if b.curPage != nil && b.curPage.Route() == p.Route() {
 		return false

ui/browsing/playlistpage.go

@@ -99,6 +99,10 @@ func (a *PlaylistPage) Tapped(*fyne.PointEvent) {
 	a.tracklist.UnselectAll()
 }
 
+func (a *PlaylistPage) SelectAll() {
+	a.tracklist.SelectAll()
+}
+
 func (a *PlaylistPage) onPlayTrackAt(tracknum int) {
 	a.pm.PlayPlaylist(a.playlistID, tracknum)
 }