(svn r5070) Merged the bridge branch
-Feature: Bridges can now be placed above:
Any railway track combination (excluding depots and waypoints)
Any road combination (excluding depots)
Clear tiles (duh), including fields
Tunnel entrances
Bridge heads

Thanks to Tron for idea and implementation, KUDr for the yapf synchronization and many others for hours of testing

There are still a number of visual problems remaining, especially when electric railways are on or under the bridge.
DO NOT REPORT THOSE BUGS FOR THE TIME BEING please.

/* $Id$ */

KNOWN BUGS / PROBLEMS:

bridges:
Graphical problems, especially in conjunction with signals and catenary

electrified rails:
Normal and elrail depots look the same. Use 'X' (transparent buildings)
   to distinguish between them
Missing curors / icons for construction (currently using the conventional ones)