Apparently this was an attempt at working around go's lack of default arguments,
but it's very unidiomatic and a bit confusing. Make it a normal parameter
instead, so all clients have to pass it explicitly.
We want to show an error when the user tries to invoke an action that expects only
a single item to be selected.
We're using the GetDisabledReason field to enforce this (as well as DisabledReason
on menu items).
I've created a ListControllerTrait to store some shared convenience functions for this.
