Rewrite our Equals implementations so they don't crash when comparing incompatible objects

OpenRA.FileFormats/Hotkey.cs

@@ -72,10 +72,8 @@ namespace OpenRA
 
 		public override bool Equals(object obj)
 		{
-			if (obj == null)
-				return false;
-
-			return (Hotkey)obj == this;
+			var o = obj as Hotkey?;
+			return o != null && o == this;
 		}
 
 		public override string ToString() { return "{0} {1}".F(Key, Modifiers.ToString("F")); }