Add singleclick and dbltap map browser events

This PR suggests that the map just relays native browser events without changing their behaviors. For example, a "click" event will be fired even the mouse moved between down and up, and two "click" events will be fired on double clicks. To still have higher level events, "dbltap" and "singleclick" events are added. "singleclick" is used in the getfeatureinfo example to prevent feature selection on double clicks (which zooms the map already).

Please review.