BUG: fixing rebase error

custom widget was removed during rebase by mistake
ENH: Porting slice projection of fiducials from Annotations

Unified the variable names.
Added an opacity slider to the widget.
ENH: make Markup fiducials the default place node

Moving toward making the Markups module replace the Annotation
fiducials, make the Markups fidicual node the default on the
mouse place tool bar.