Updating XmlRpc functionality to clear up the confusion with Live Writer (WLW) and categories (since there is not category support)

- updated IXmlRpcHandler to also have a vvoid SetCapabilities(XElement element) so handlers can specify what capabilities they support
- updated the WLW manifest to have a thiner capability set (e.g. no category support) so handlers can light up what they support
- updated the XmlRpcHandlers to add what capabilities they support. there'a room for imporoving (adding/removing) specified capabilities here so the WLW only shows features for what's supported
- added WLW support for tags so a tags input can be used in place of the categories input in the WLW UI

--HG--
branch : dev

src/Orchard.Web/Modules/Orchard.Media/Services/XmlRpcHandler.cs

@@ -1,6 +1,7 @@
 using System;
 using System.IO;
 using System.Web;
+using System.Xml.Linq;
 using JetBrains.Annotations;
 using Orchard.Core.XmlRpc;
 using Orchard.Core.XmlRpc.Models;
@@ -18,6 +19,11 @@ namespace Orchard.Media.Services {
             _authorizationService = authorizationService;
         }
 
+        public void SetCapabilities(XElement options) {
+            const string manifestUri = "http://schemas.microsoft.com/wlw/manifest/weblog";
+            options.SetElementValue(XName.Get("supportsFileUpload", manifestUri), "Yes");
+        }
+
         public void Process(XmlRpcContext context) {
             var uriBuilder = new UriBuilder(context.HttpContext.Request.ToUrlString()) {
                 Path = context.HttpContext.Request.ApplicationPath,