[Suggestion]: Slot improvements

[Lorith]

A few things that I noticed would be useful while working with large numbers of slots:

Slot groups -- Even if only for organization, this would be extremely useful. If you have several dozen slots in use, it can be difficult to find the particular one you want, and if you are organizing things into clusters that get used by the same vehicles, it would be very handy to be able to put all the slots they need into a group so you can find them all easily. Optionally, include releasing a group to release all slots it contains. Release all slots order -- I often have trains in multiple slots such as X cluster + Y loading, and it would be convenient to be able to release all slots at once when I sent them to unload rather than having to do so one by one

[JGRennison]

I can see that a way to group slots in the list would be useful for organisation. I can take a look when I've got some time available.

I am quite reluctant to add a release all slots order, because this is a very tempting footgun when you're also using slots attached to signals, and you'd effectively lose the property that different slots are orthogonal to each other. Adding some sort of release all slots in slot group order would not seem quite so problematic though.

[Lorith]

I had completely forgotten about signals being able to interact with slots since I had some user error issues with that and stopped using them. Very good point, I can immediately see multiple cases that release all would be able to cause problems. I am glad you pointed that out, or I would have completely missed it.