UI.ListFleets x:Int, y:Int, [flags:Any, ok:Str, heading:Str]
Choose a fleet. Lists all fleets at the specified x,y. If the user chooses one, returns their Id in UI.Result. If the user cancels the dialog using ESC, UI.Result is set to EMPTY.
The flags parameter modifies the function's behaviour:
- "a": list all fleets, ignore x,y.
- "e": do not display a dialog if there's just one fleet.
Since: PCC2 1.99.17
