Support of custom classes with custom geographical coordinates properties and content

This issue is about making it possible to use the map application in the following context:

• A class "MyClass" in the wiki has several properties including one for latitude and one for longitude, named for instance "lat" and "long".
• The wiki contains many objects of that class, with existing values for "lat" and "long".
• The user wants to represent all these objects a the map, and a specific content property that describes each entity.

This means the map app would include the possibility to declare the name of a custom class and custom properties to be used for obtaining the coordinates of markers and their content. The default would be "Maps.Code.PointClass" and its geo and content properties, but it could be customized.