Revert "Simplifying layer visibility handling"

This reverts commit f0ef4728d8bc711d5840198507274bb720397a88.
Temporarily hide viewPortDiv during container shuffling

This is needed to avoid both the GMap and the OpenLayers map to be visible
at the same time.
Safeguard against removed/destroyed map
Removing some lint and fixing docs
Fixing visibility issue reported by @TobiasLohner
More checks before calling clearListeners
Merge pull request #987 from ahocevar/google-orphaned

Vector layer duplicates when changeing to google base layer. r=@bartvde