Allow player to add a "priority unit" to production queue

* If production is ordered by `Ctrl + Left Click` the item is added to the top of the stack after the currently produced item
* Works with `Shift` for priority queueing of 5 items
* This modifier is not taken into account for `ParallelProductionQueue` as it doesn't make sense in that context

OpenRA.Mods.Common/Traits/Player/ProductionQueue.cs

@@ -406,7 +406,7 @@ namespace OpenRA.Mods.Common.Traits
 								else if (!hasPlayedSound && time > 0)
 									hasPlayedSound = Game.Sound.PlayNotification(rules, self.Owner, "Speech", Info.BlockedAudio, self.Owner.Faction.InternalName);
 							}
-						})));
+						})), !order.Queued);
 					}
 
 					break;
@@ -470,9 +470,12 @@ namespace OpenRA.Mods.Common.Traits
 			Queue.Remove(item);
 		}
 
-		protected void BeginProduction(ProductionItem item)
+		protected virtual void BeginProduction(ProductionItem item, bool hasPriority)
 		{
-			Queue.Add(item);
+			if (hasPriority && Queue.Count > 1)
+				Queue.Insert(1, item);
+			else
+				Queue.Add(item);
 		}
 
 		public virtual int RemainingTimeActual(ProductionItem item)