Make building custom names accessible

[JLBShecky]

Buildings such as the radar, sience lab, and particularly the train stop all have custom name, but they don't seem to be accessible via lua. Although the main thing that I see getting the custom names from entities being used for is to get the list of train stops, for dynamic train routing I am sure that other uses for getting the custom name could be applied, for example scenario text that references the structure custom name, although that might be better hard coded come to think of it.

Anyways the ability to read an entities' custom name would be useful in my own personal endeavors.