New slider Range parameter. Palette modifications. Potential crash fix. Clamp function.

Range parameter added to slider. Supports returning a range of values
rather than just 0-1. Allows you to not have to post process the offset.
Modified palette selector to not have full range, which was causing
blown out units.
Introduced exension method Clamp<T>(min, max)
Fixed crash deserializing out of bound color value using above
extension.

OpenRA.FileFormats/Exts.cs

@@ -85,5 +85,15 @@ namespace OpenRA
 		{
 			return (T[])mi.GetCustomAttributes( typeof( T ), true );
 		}
+
+		public static T Clamp<T>(this T val, T min, T max) where T : IComparable<T>
+		{
+			if (val.CompareTo(min) < 0)
+				return min;
+			else if (val.CompareTo(max) > 0)
+				return max;
+			else
+				return val;
+		}
 	}
 }