replace f8c99edb's dynamic casts by just-as-ugly static casts

add TransportCAI::IsBusyLoading and call that instead of LoadStillValid
(which returns true when called outside of TransportCAI::ExecuteLoading
even if the transport just has a CMD_MOVE + CMD_SET_WANTED_MAX_SPEED in
its queue)

make TransportCAI 0.01% more readable
Merge branch 'develop' of github.com:spring/spring into develop