Let BoxLayout.add_widget use the ``canvas`` argument
Let BoxLayout.add_widget use the ``canvas`` argument

This is pretty much the same as #5715 but for BoxLayout.

I looked through the other Layout subclasses apart from in
the examples and this seems like the only remaining one
with a custom ``add_widget`` that didn't take ``canvas``.

rnixx/kivy,matham/kivy,LogicalDash/kivy,akshayaurora/kivy,kivy/kivy,matham/kivy,matham/kivy,rnixx/kivy,akshayaurora/kivy,rnixx/kivy,inclement/kivy,matham/kivy,LogicalDash/kivy,inclement/kivy,LogicalDash/kivy,akshayaurora/kivy,inclement/kivy,kivy/kivy,LogicalDash/kivy,kivy/kivy