Validation, settings UI cleanup and different settings models, oh my

New: Download client UI matches other settings
Fixed: Prevent drone factory folder from being set to invalid paths/root path for series
Fixed: Switching pages in settings will not hide changes
Fixed: Test download clients
Fixed: Settings are validated before saving

src/NzbDrone.Core/Validation/FolderValidator.cs

@@ -0,0 +1,19 @@
+using FluentValidation.Validators;
+using NzbDrone.Common;
+
+namespace NzbDrone.Core.Validation
+{
+    public class FolderValidator : PropertyValidator
+    {
+        public FolderValidator()
+            : base("Invalid Path")
+        {
+        }
+
+        protected override bool IsValid(PropertyValidatorContext context)
+        {
+            if (context.PropertyValue == null) return false;
+            return context.PropertyValue.ToString().IsPathValid();
+        }
+    }
+}