Add core widget base class providing module semver ranges

[SylvainCorlay]

Closes #1002.

On the Python side, I use multiple inheritance to specify the version range for all core widgets without impacting the base Widget and DOMWidget classes that are used in third-party libraries.

On the JS side, I still need to make the change.

Proposed solution: use '^' + require('../package.json').version

@jasongrout thoughts?

[jasongrout]

Delete this line?

[jasongrout]

Proposed solution: use '^' + require('../package.json'),version

That sounds consistent to me.

[jasongrout]

Maybe this?

`^${require('../package.json').version}`

[SylvainCorlay]

Maybe this?
^${require('../package.json').version}

This is cool :)

[SylvainCorlay]

So on the js side, I have CoreWidgetModel and CoreDomWidgetModel since default() will not work well with multiple inheritance.

[SylvainCorlay]

So if the semver req specified by the backend for a model is ^4.3.0, and the installed/available js version is 4.4.3, what happens when the backend opens a widget, is that the value for model_module_version stored in the model is ^4.3.0.

When we save the widget state, this value is different from the js-side default (^4.4.3) and therefore is the one used in the serialization.

An idea would be to have a custom serializer for these attributes to restrict the semver range.

[SylvainCorlay]

@ellisonbg @jasongrout

[SylvainCorlay]

After in-person discussion with @jasongrout on this, we decided that this was still the right approach.