Viewer mixins

[mramato]

Note: This is a pull into the viewer branch.

We can do final cleanup in the viewer branch, but open a separate pull so we can make sure we all agree on the approach.

@shunter Take a look at this and merge if you're happy.
@emackey Your comments are also welcome (obviously so are everyone else's).

[mramato]

I ended up submitting some other cleanup to this branch as well, so the sooner we can agree and get this back into the viewer branch, the better.

[emackey]

I like the mixins 👍 mostly because it makes the code look as if the widget had the functionality built-in, while still providing an option for the same widget without the functionality.

I asked this in person, but are we sure all of this widget's functionality shouldn't just be a mixin for the base CesiumWidget? Probably a lot of lost work if we change that plan now, but are we going to decide to do that down the road?

[shunter]

The Viewer widget doesn't have much functionality beyond composing the sub-widgets. One issue with making the sub-widgets mixins too is that we'd need some kind of notification system, e.g. the DynamicObject mixin depends on the homeButton widget, if it exists.

I think this is a good start. We can tweak later if we need to. Merging.