Loosen actor type on PopupMenuSection

[AmionSky]

PopupMenuSection does set actor to a St.BoxLayout in its constuctor, it does not use it (nor any other code assumes it as a St.BoxLayout), but if I want to modify it in a subclass, it is now limited to subclass of St.BoxLayout instead of Clutter.Actor (which is from the parent class).

I would like to loosen this requirement. Especially since the only thing it does is

this.actor = this.box;

and the St.BoxLayout is still accessible via this.box property