Basic railroad track map tiles

This adds some new terrain types and (over)map tiles to C:DDA. Those are all railroad-related.

New terrain types are: railroad tie (several directions), railroad track, buffer stop, railway crossing signal, crossbuck (wooden or metal).
New map tiles are: Straight rail, curved rail, rail ending with buffer stop, railway crossing (with crossbuck or railway crossing signal) and 2 kinds of railroad bridges.
The railway crossing signal, buffer stop and the crossbucks are smashable for wood or metal. The signal drops cheap electronics.

A (not accurate anymore) preview is here: http://smf.cataclysmdda.com/index.php?topic=5678.msg103528#msg103528

Please note there are no junctions for railroad tracks; these have been blacklisted.

To test, open the debug menu’s map editor and try everything with the word “rail” in it.