OverviewMap widget should support <layer/> as in the main config.xml

[jcfranco]

We could reuse some of the utilities we have to implement this and clean up OverviewMapComponent.

If we do reuse the same logic for handling the main config's layers, it will change the OverviewMap widget's default layer visibility. The layer visibility is true by default in the OverviewMap widget and false for the main config's layers. We could either:

• Document that the default layer visibility is now the same as described in the layer tag reference.
• Maintain the old behavior in the widget

Note: This only applies when is defined in the widget's config.