Added shunt panel controls. Combo box updates when a train is added to
trainInfo. Changed each train to a panel and updated button and panel
enable/disables. Shunt will currently remove the selected carriages, but
still need to implement spur line functionality. Also need to implement
conductor still.

P.S. we might need two panels in trainInfo, one for spur line as well.
Or I could just add a spur line panel down the bottom of trainInfo.