Add stopEvent option in ol.OverlayOptions typedef
Add new container which don't stop event propagation

and allow overlays to optionnally stop event propagation
Adjust event propagation for overlay example
Adjust event propagation for popup example
Add controls to ol-overlaycontainer-stopevent

so that events won't be propagated to the map when interacting with a control
Add insertFirst option in ol.OverlayOptions typedef
Insert or append overlay based on insertFirst option

This is to fix z-index issues with overlays that pass above ol3 default
controls.
Fix overlay z-index issue in popup example
Fix overlay z-index issue in overlay example
Set z-index: auto so that popup is not above controls