Merge viewerEntityMixin into Viewer

Deprecated `viewerEntityMixin` and moved it's functionality into `Viewer.
Clean up Viewer code after merging in viewerEntityMixin.
Remove deprecated calls to viewerEntityMixin.
Merge remote-tracking branch 'origin/master' into merge-viewerEntityMixin
Merge remote-tracking branch 'origin/master' into merge-viewerEntityMixin

Conflicts:
	CHANGES.md
Make FullscreenViewModel accept element id

Everywhere in Cesium we take an element, we also allow the string id to
be used instead. This just updates `FullscreenViewModel` to be in line with
everything else,
Merge remote-tracking branch 'origin/master' into merge-viewerEntityMixin
Merge pull request #2339 from AnalyticalGraphicsInc/fullscreen-element

Make FullscreenViewModel accept element id
Fix WMS picking.

Also fix URL used in WMS Sandcastle example.
Merge pull request #2325 from AnalyticalGraphicsInc/merge-viewerEntityMixin

Make viewerEntityMixin part of Viewer proper
Merge remote-tracking branch 'origin/master' into display-condition