Added DriverAidWindow, accessible (tentatively) using F11. The DriverAid is the first step in developing a train control system. For now, the DriverAid system is intended to simply assist the driver in knowing exactly what speed they should be going, and warns them if they are going too fast.
Currently, the speedometer works (although it's calibrated in kmh only). In future, other features will be enabled.
git-svn-id: http://svn.uktrainsim.com/svn/openrails/trunk@489 a9f3cfff-6c41-7446-b0ca-052bd761e6eb