ResultVectorLayer - 'beforefeatureadded' event listener

[adube]

Currently, in the ResultVectorLayer, features are validated first, then added to the layer.

To allow external widgets to use the same validation, we could :

• add the features first
• register a vector layer 'beforefeatureadded' event to check if a feature is valid upon addition and make the callback method return 'false' if the feature isn't valid.

Also, in the same way, we could :

• register a vector layer 'featureadded' event to check if the feature added was the first. If it was linked to a resource, then set the 'singleResource' and 'multiQueryResource' (QueryOnClick) properties.

[adube]

commited in r1025.