Custom widget persistence

[SylvainCorlay]

• Currently, there is the following bug in the widget persistence.

If Foobaris a custom widget that has a model defined in a requirejs module, specified in the _model_module widget attribute, then something like HBox([Foobar()]) won't survive a page reload.
However, Foobar() alone does persist when it is not a child of a container.

In short, custom widgets having custom models don't persist when they are children other widgets.

The symptom in the js console is the load_class complaining that undefined is not in the registry.

• The reason is that that the creation of child widget views is done by the parent widget, at a time when their state, including _view_name and _view_module has not yet been set in the model.

Hence I changed the widget persistence mechanism, to first fetch the states of all the persisted models before instantiating any view.

Besides, the code is quite simpler, and it significantly reduces the line count.

• Finally, this should probably be backported to 3.x.

[SylvainCorlay]

@jasongrout @jdfreder

[jasongrout]

FYI, the first three hunks and the last hunk are just whitespace changes. Real changes are just lines 417-465.

[jasongrout]

At a glance, this looks okay. @jdfreder, since you wrote the original code, can you take a look at this too? Thanks!

[jdfreder]

Thanks @SylvainCorlay !