Allow left click to cancel AttackMove without clearing selected units.

OpenRA.Game/Orders/GenericSelectTarget.cs

@@ -68,5 +68,7 @@ namespace OpenRA.Orders
 			// Custom order generators always override selection
 			return true;
 		}
+
+		public override bool ClearSelectionOnLeftClick { get { return false; } }
 	}
 }

OpenRA.Game/Orders/UnitOrderGenerator.cs

@@ -204,5 +204,7 @@ namespace OpenRA.Orders
 				Target = target;
 			}
 		}
+
+		public virtual bool ClearSelectionOnLeftClick { get { return true; } }
 	}
 }