Add the command identifier to GetHighlightCommand

[Garanas]

The GetHighlightCommand global was introduced by #65 and is in use by the Context Based Templates feature. It's a great addition, but it's missing one field: the command identifier. It's the value that we can give to the global DeleteCommand to cancel the command.

With it we can introduce a feature to 'swipe to delete'. That way you can first copy a build command using the context based templates feature, swipe to delete the original and then replace the command. This would allow us to create a setup for FAForever/fa#5982.

As an example, the command identifier is returned by UserUnit:GetCommandQueue.