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
Merge remote-tracking branch 'origin/master' into merge-viewerEntityMixin
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