Viewer toolbar buttons should be display:none when hidden

Please see [this example](http://cesiumjs.org/Cesium/Apps/Sandcastle/?src=Hello%20World.html&label=Showcases&gist=9a000735ebddff50b4668998a6dea784) -- mouse over the red area and you should see a crosshair. Note that when the mouse moves under the scene mode selector, there are two small areas where the crosshair turns back to a normal pointer. This is because the scene mode buttons for 2D and Columbus are not actually hidden, they are still there with an opacity of 0.

I understand why this is being done -- `display:none` doesn't play nice with CSS animations -- but once the animation is done, these buttons need to be removed from the document flow because they can obscure items below the toolbar. I've contrived a simple injected element in this example, but it can also prevent clicks within e.g. the InfoBox widget in some situations.

I don't think I understand how your animations line up well enough to contribute a patch, but hopefully there's enough information in this ticket that your Animation Expert can quickly fix it.

Start with the linked Sandcastle example and inspect the scene mode selector toolbar and its hide/show animation behavior. Confirm which buttons remain in the document flow after hiding; done means hidden buttons no longer intercept pointer movement or clicks, while the toolbar animation still works.

Written by the indexing model from the issue text.