Allow Multiple Labels for a Layer

[geographika]

This PR allows mutliple label layers to be configured in the config JSON.

The name should match a back-end WMS layer containing labels. title will be displayed in the sub-menu, similar to the styles property.

E.g.

 "labels": [
     { "name": "LabelsName", "title": "Names" },
     { "name": "LabelsCode", "title": "Codes" }
 ]

This will create a new context menu option with a submenu allowing different labels to be selected.

The labelClassName remains, but if both are set a warning will be given and labels takes priority.