Auto-detect map object type

[jfournier]

With the new ApplicationDefinition structure, each WidgetSet corresponds to a single map object, which may refer to a Map by id. A Map contains Layer tags that each have a type that corresponds to an OpenLayers type. Right now, the Map widget in the WidgetSet is still one of MGMap or MSMap. We'll need to make changes, at some point, to grab the layers from the Map and create the right OpenLayers Layer objects for them. I'd also like to consider how we could expose some sort of API to simplify (or make possible) adding a new Layer at runtime.