Scene mode widget

I'm opening this pull request to get some early feedback and make collaboration with  @emackey a bit easier.  I don't expect this to be merged for a little while.  Feel free to try it out and provide feedback at any time however.

Main changes to this request so far are:
* A new single-button `SceneModeWidget` for switching among scene modes.
* Added `onTransitionStart` and `onTransitionComplete` events to `SceneTransitioner`, I also exposed the ability to complete a morph early via a call to `endMorph`.
* Make `CesiumViewerWidget` use the new widget.

There are a few things that still need to be done.
* [ ] The current new SVGs are placeholders and I've asked @emackey to go hog wild on the CSS and images associated with the control.  The functionality itself is complete, but the look needs a lot of work.  The layout within CesiumViewerWidget is also bad.
* [ ] I was going to go through and document `SceneTransitioner`, but I hesitated because I've heard in the past that it's "all going to change".  Is this still true?  @pjcozzi should I both to document it?
* [ ] I was thinking about giving the same drop-button style treatment to the current `imagery` and `home` buttons, this way we can unify the look and feel.  This would also make it possible to completely remove all Dojo from the current `CesiumViewerWidget` implementation.