[2.0] Add ability to add shared interrupts to ungrouped trains

[MrBurd]

I'd like to be able to set interrupts to ungrouped trains; their schedules are all different, but I do want all of them to use a 'refuel' interrupt.
Setting each of these manually is annoying and it's definitely possible that I'll forget this and a train will run out of fuel.

Train groups are very tailored towards a specific use case (using trains like logistic trains) and not everyone wants to use that. Especially early game, I don't see a point for groups since there's many point to point trains dedicated to just 2 stations, so schedules shouldn't be shared.
But I do want all trains to use the same interrupt, especially an obvious one like refueling.
So I'd like to see some way to apply one interrupt to many trains.

This might be done using a 'default' group for ungrouped trains; simply allow us to set interrupts for the default group as well.

Another way this can be done is by adding a setting to a group that makes only interrupts shared, not schedules, ie. "[checkbox, checked by default] Share schedules between trains in group". Uncheck this and only the interrupts will be shared.

[Necandum]

Second.