In order to transition to Gtk+3, we need to replace some deprecated widgets like GtkAlignment.
This bug was referenced in this commit: https://git.xfce.org/xfce/xfce4-panel/commit?id=3554f9397a638e8188ee950d9e4562c41c60a367
This bug was referenced in this commit: https://git.xfce.org/xfce/xfce4-panel/commit?id=998a2e96b912a96ce4f475bee099b1b0f69d7274