add virtual train signals and support for it in the dispatcher

[mrvn]

This patch adds virtual signals for combinations for all (not hidden) locomotives and wagons. If there are multiple prototypes for locomotives or wagons (per type) then an "any" signal is also created. The dispatcher is extended to dispatch trains matching those signals to provider and requester stations with a special schedule for train-signal trains waiting for the matching signal. Artillery trains get extra conditions to leave on full cargo and empty cargo or 10s inactivity.