UI: Add the ability to change a DualSense/DualShock 4's LED color.

Not functional yet. This is the UI & persistence side of #572.

src/Ryujinx.Input/GamepadFeaturesFlag.cs

@@ -24,5 +24,10 @@ namespace Ryujinx.Input
         /// <remarks>Also named sixaxis</remarks>
         /// </summary>
         Motion,
+        
+        /// <summary>
+        ///     The LED on the back of modern PlayStation controllers (DualSense &amp; DualShock 4).
+        /// </summary>
+        Led,
     }
 }