func_useableladder in-game tut

Illustrates the construction of a func_useableladder with in-game messages, inspired by Captain P's HL in-game tut and Valve's sdk_xxx series of vmfs. A full tutorial on a construct like this seems like too much.

Run the bsp, stand on the arrows to view the left-to-right construction of a ladder leading to a "rooftop."

Includes a couple of tips for making smooth ladder mounts and dismounts.

Use the Messages visgroup in the vmf to hide the point_messages and get a better view of the other entities.