Add Track::get_endpoint_position to avoid duplicating calculations

Add Track::get_endpoint_position to avoid duplicating calculations
Fix Block::traverse so it won't hang
Fix memory leaks reported by valgrind
Add signal_speed_changed for locomotive
Bugfixes for block reservation of trains
Color blocks that are occupied by a train
Remove some unnecessary stuff from Engineer
Add simulation mode for sensors
Finish train placing
Add speed display to TrainPanel