Only set the search box cue banner on Windows

This particular invocation of SendMessage causes an exception under mono
on Linux.

Opened issue #209 for getting this working properly under Linux.

Controls/SearchBox.cs

@@ -1,6 +1,6 @@
 /*
  * This file is part of Radio Downloader.
- * Copyright © 2007-2012 by the authors - see the AUTHORS file for details.
+ * Copyright © 2007-2016 by the authors - see the AUTHORS file for details.
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -94,7 +94,11 @@ public string CueBanner
             set
             {
                 this.cueBanner = value;
-                NativeMethods.SendMessage(this.textBox.Handle, NativeMethods.EM_SETCUEBANNER, IntPtr.Zero, this.cueBanner);
+
+                if (OsUtils.Windows())
+                {
+                    NativeMethods.SendMessage(this.textBox.Handle, NativeMethods.EM_SETCUEBANNER, IntPtr.Zero, this.cueBanner);
+                }
             }
         }