Notes on the implementation of Media Overlays validation:

• The OverlayChecker#validate() method currently only verifies the publication's SMIL file(s), it should also verify the OPF metadata specific to Media Overlays (total duration == sum of individual spine items duration, CSS "active" class checks?).
• The OverlayHandler class doesn't verify durations (clip-begin / end pairs), it should also cross-check with the corresponding OPF metadata.
• The OverlayCheckerFactory doesn't need to do anything else.