playlistcontrols: Avoid a resize when the song count is updated

When a new playlist is selected from the PlaylistViews' sidebar, the songs_count_label is hidden while it is updated. This PlaylistControls is resized which results in a strange effect: the circular button on the right position changes two times (one time when the label is hidden and a second time when the label is visible again).

Fix the issue by adding an empty label beside songs_count_label. This allows to preserve the height when the song count is updated.