Add ability to control which widgets are created at construction time.  Also properly implement destroy and clean up some Timeline code to allow it to be destroyed.

Conflicts:
	Source/Widgets/BaseLayerPicker/BaseLayerPicker.js
	Source/Widgets/FullscreenButton/FullscreenButton.js
	Source/Widgets/lighter.css
	Source/Widgets/widgets.css

This change primarily adds back functionality to the Cesium Viewer that is now considered app specific, in this case, the existing query parameters.  Also made a few other tweaks.

1. Replace all usage of CesiumViewerWidget with the new Viewer widget.
2. Delete `Client CZML` example becasue it's no longer needed and redundant with `Simple CZML Demo`
3. Delete `Two Viewer Widgets` because we aren't ready to support multiple scenes the correct way.  The technique used by this demo resulted in poor performance and lots of data duplication.

Conflicts:
	Specs/DynamicScene/DataSourceDisplaySpec.js

Conflicts:
	Apps/CesiumViewer/CesiumViewer.js
	Apps/Sandcastle/gallery/Simple CZML Demo.html
	Apps/Sandcastle/gallery/Terrain.html
	Source/Widgets/Dojo/CesiumViewerWidget.js
	Source/Widgets/FullscreenButton/FullscreenButton.js

1. Make the new Viewer follow the new API style of other widgets.
2. Fix Sandcastle examples so that they all run.
3. Fix specs.

Conflicts:
	Apps/CesiumViewer/CesiumViewer.js
	Apps/Sandcastle/gallery/Simple CZML Demo.html
	Apps/Sandcastle/gallery/Terrain.html
	Source/Widgets/Dojo/CesiumViewerWidget.js

Conflicts:
	Source/Widgets/Dojo/CesiumViewerWidget.js

Conflicts:
	Source/Widgets/Dojo/CesiumViewerWidget.js

…en included in Sandcastle.

Conflicts:
	Specs/Widgets/CesiumWidget/CesiumWidgetSpec.js

To keep Viewer as lean and flexible as possible, drag and drop code is now it's own utility class, ViewerDropHandler, which allows users to add drag & drop support in one line of code.

Refactor camera tracking code into a common ViewerDynamicSceneControls utility class, which will let users using the DyanmicScene layer easily add useful input behavior.

Conflicts:
	Source/Widgets/Dojo/CesiumViewerWidget.js

…ice twice on startup.

Viewer mixins

Conflicts:
	CHANGES.md

Also expose the flight duration property on HomeButtonViewModel.

Also fix some error handling and query string parsing code.  We'll have more robust notification in the Viewer widget with future work.

This still isn't perfect, but it's as good as the old Viewer widget and has the added bonus of the timeline and logos moving accordingly depending on whether the animation/timeline widgets exists.

…ime.