Move the layer menu button on the left side
Currently we have in the layer controls a button group with two buttons for disabling audio/video in the layer and near it a menu button for moving the layer etc.
It would look better if the menu button is moved on the very left of that row, before the layer name entry.
See the LayerControls
constructor: https://gitlab.gnome.org/GNOME/pitivi/-/blob/d92781acefc2b327e4d09748a33e3f672ca28831/pitivi/timeline/layer.py#L58