Feature: Build train locomotive filter #8733
Motivation / Problem
When creating a train, there is no way of filtering by engines, given that the 'None' cargo filter removes all engines that also carry cargo, typically passengers and mail. As a player, I want to be able to see all the available engines regardless of whether they can carry cargo or not. This is especially helpful when creating passenger trains.
This PR adds a new filter in the train vehicle window drop down menu so in addition to (1) any cargo, (2) no cargo, and (3) a specific cargo, players can also filter by engines only, regardless of whether they carry cargo or not.
This is how it looks, observe that some of the engines can carry passengers, like the selected one:
seems a little hacky to me, so I just reused the same pattern using CT_AUTO_REFIT as there wasn't a better option.
Checklist for review
Some things are not automated, and forgotten often. This list is a reminder for the reviewers.