Cameras 2 and 3 moving along train

With commands Shift-Alt-7(Numpad) and Shift-Alt-1(Numpad) cameras 2 or 3 move gently forward or backward along a stopped or moving train. By pressing a second time the key sequence the motion stops. The usual commands for cameras 2 and 3 remain all available. The new commands are integrated with the already available ones.

Discussion: http://www.elvastower.com/forums/index.php?/topic/32737-cameras-2-and-3-moving-along-train/
Roadmap: https://trello.com/c/i9Xz71MB/401-moving-camera-2-and-3-smoothly-along-train