Make Viewer handle data source clock management.

Previously, viewerDragDropMixin, CesiumViewer, and Sandcastle examples all had code to set the display's clock to match the first loaded data source.  Now, Viewer does that automatically by listening to the DataSourceCollection's event.

Viewer now owns its DataSourceCollection, instead of DataSourceDisplay owning it.

Also rename "Simple CZML Demo" Sandcastle example to "CZML" since there is no other CZML example anymore.