Simplify the interaction with overlays by adding an OverlayManager
Convert the existing overlays to use the OverlayManager
Update overlay stubs for the B2G viewer
Merge pull request #4823 from Snuffleupagus/overlay-manager

Simplify the interaction with overlays by adding an OverlayManager
Small comment fix
Merge pull request #4850 from timvandermeij/comment-nit

Small comment fix