Add Box (and HBox and VBox) to dispatching to contents based on type

[danielballan]

A Box model will hold a list of heterogenous models, as in [figure_spec, slider_spec, figure_spec_list]. Each Box view will iterate this list and build the corresponding view for its framework.

We may wish to make it possible to inject custom classes for the sub-views, as in:

VBoxView(vbox_model, views={bluesky_widgets.models.FigureSpec: CustomQtFigureView})

where these override the defaults. We could also, as usual, accepts strings with an import path in place of the actual classes.

We should also pay attention to the __mro__. We should check if the specific model class has a specific view, and then walk its base classes in order to check them.