Motivation

This is a possible addition to #7028 to allow (some) contextual actions when vehicle lists are grouped by shared orders. Contextual actions are clicks on a vehicle in the vehicle list that complete some other ongoing action, such as to clone a vehicle or to copy/share orders.

This brings the option with grouping enabled to "parity" with the non-grouped option, by supporting contextual actions wherever it is unambiguous.

Description

The main addition of this PR is the contextual callback (VehicleClicked()) overload for multiple vehicles. This PR is implemented as a generic mechanism for contextual actions on sets containing multiple vehicles, instead of just a single vehicle. It hence does not rely on anything specific to grouping by shared orders, so it will still work even if in the future we have other ways to group vehicles.

I've implemented this whole feature outside of the DoCommand system. I'm not sure if this is the best way, but since vehicle grouping is a purely UI feature (kind of a "view" over the real list of vehicles), it might be arguable that this resolution of contextual actions should also happen outside of the command system too. Also I'm not sure if there's a simpler way to implement this.

How to test

1. Load a game, open the vehicle list of some station with many routes passing through it, and set the vehicle list to group by shared orders.
2. Create a new vehicle, open the order list, click "Go to", then click any row in the vehicle list from step 1. Observe that the order list is copied to the new vehicle.
3. Same as step 2, but hold down CTRL when clicking the row in the vehicle list. Observe that the new vehicle is now sharing orders.
4. Create a depot, click "Clone Vehicle", then click any row in the vehicle list from step 1. If all vehicles in the vehicle list have the same sequence of engines, then the vehicle will be cloned. Otherwise an error will be displayed.
5. Same as step 4, but hold down CTRL when clicking the row in the vehicle list. You should observe the same thing as step 4, but the new vehicle is now sharing orders.

Note: I'm not sure if checking that two vehicles have the same sequence of engines is the correct way to check if two vehicles are "equal" (for steps 4 and 5). Please correct me if it isn't!