(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.

ai/default/default.c

@@ -2158,9 +2158,7 @@ static bool AiRemoveTileAndGoForward(Player *p)
 				return false;
 			p->ai.cur_tile_a = TILE_MASK(_build_tunnel_endtile - TileOffsByDir(p->ai.cur_dir_a));
 			return true;
-		}
-
-		if (IsBridgeRamp(tile)) {
+		} else {
 			// Check if the bridge points in the right direction.
 			// This is not really needed the first place AiRemoveTileAndGoForward is called.
 			if (DiagDirToAxis(GetBridgeRampDirection(tile)) != (p->ai.cur_dir_a & 1U)) return false;
@@ -3692,7 +3690,6 @@ pos_3:
 	} else if (IsTileType(tile, MP_TUNNELBRIDGE)) {
 		if (!IsTileOwner(tile, _current_player) ||
 				!IsBridge(tile) ||
-				!IsBridgeRamp(tile) ||
 				GetBridgeTransportType(tile) != TRANSPORT_RAIL) {
 			return;
 		}