Use ol.LEGACY_IE_SUPPORT and ol.IS_LEGACY_IE define

See #1605
Unset z-index in overlay example
Merge pull request #2692 from fredj/overlay-index

Unset z-index in overlay example
Change signature of ol.layer.Group#getLayers

This commit changes the signature of ol.layer.Group#getLayers from {ol.Collection.<ol.layer.Base>|undefined} to {!ol.Collection.<ol.layer.Base>}. In this way the caller is guaranteed that getLayers returns a dereferencable object.
Remove undefined from ol.Map#getLayers return type

It's already assumed that the collection is defined in ol.Map#addLayer
and ol.Map#removeLayer.
Merge pull request #2675 from fredj/map.getlayers

Remove undefined from ol.Map#getLayers return type
Merge pull request #2687 from fredj/legacy-ie

Use ol.LEGACY_IE_SUPPORT and ol.IS_LEGACY_IE define
Remove unnecessary newlines
Remove unnecessary parentheses
Fix documentation typo