Implement experience multipliers.

OpenRA.Mods.Common/Traits/Multipliers/GainsExperienceMultiplier.cs

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+#region Copyright & License Information
+/*
+ * Copyright 2007-2017 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, either version 3 of
+ * the License, or (at your option) any later version. For more
+ * information, see COPYING.
+ */
+#endregion
+
+using OpenRA.Traits;
+
+namespace OpenRA.Mods.Common.Traits
+{
+	[Desc("Modifies the experience gathered by this actor.")]
+	public class GainsExperienceMultiplierInfo : ConditionalTraitInfo
+	{
+		[FieldLoader.Require]
+		[Desc("Percentage modifier to apply.")]
+		public readonly int Modifier = 100;
+
+		public override object Create(ActorInitializer init) { return new GainsExperienceMultiplier(this); }
+	}
+
+	public class GainsExperienceMultiplier : ConditionalTrait<GainsExperienceMultiplierInfo>, IGainsExperienceModifier
+	{
+		public GainsExperienceMultiplier(GainsExperienceMultiplierInfo info)
+			: base(info) { }
+
+		int IGainsExperienceModifier.GetGainsExperienceModifier()
+		{
+			return IsTraitDisabled ? 100 : Info.Modifier;
+		}
+	}
+}