Removes dtgePreview from the RouteData values

This was a lazy design choice.

Updated developer guide documentation to use `UmbracoContext.InPreviewMode` instead.

Closes #123

Author: leekelleher

docs/developers-guide.md

@@ -139,11 +139,11 @@ Because we treat your data as a standard `IPublishedContent` entity, that means
 
 #### Rendering Alternative Preview Content
 
-If your front end view is rather complex, you may decide that you want to feed the back office preview an alternative, less complex view. To do this, within your Razor view/partial, check for a RouteData parameter `dtgePreview` being set to true to detect being in preview mode to provide an alternative view.
+If your front end view is rather complex, you may decide that you want to feed the back office preview an alternative, less complex view. To do this, within your Razor view/partial, check `UmbracoContext.InPreviewMode` is set to true to detect being in preview mode to provide an alternative view.
 
 ```
 @inherits Umbraco.Web.Mvc.UmbracoViewPage
-@if (ViewContext.RouteData.Values["dtgePreview"])
+@if (UmbracoContext.InPreviewMode)
 {
 	// Render preview view
 }

src/Our.Umbraco.DocTypeGridEditor/Web/Controllers/DocTypeGridEditorSurfaceController.cs

@@ -1,6 +1,4 @@
-using System;
-using System.Web.Mvc;
-using Our.Umbraco.DocTypeGridEditor.Extensions;
+using System.Web.Mvc;
 using Our.Umbraco.DocTypeGridEditor.Web.Helpers;
 using Umbraco.Core;
 using Umbraco.Core.Models;
@@ -31,7 +29,7 @@ public string PreviewViewPath
 
         public bool IsPreview
         {
-            get { return ControllerContext.RouteData.Values.TryGetValue("dtgePreview", out object value) && Convert.ToBoolean(value); }
+            get { return UmbracoContext.InPreviewMode; }
         }
 
         protected PartialViewResult CurrentPartialView(object model = null)

src/Our.Umbraco.DocTypeGridEditor/Web/Extensions/HtmlHelperExtensions.cs

@@ -1,7 +1,6 @@
 using System.Web;
 using System.Web.Mvc;
 using System.Web.Mvc.Html;
-using Our.Umbraco.DocTypeGridEditor.Extensions;
 using Our.Umbraco.DocTypeGridEditor.Web.Helpers;
 using Our.Umbraco.DocTypeGridEditor.Web.Mvc;
 using Umbraco.Core;
@@ -47,8 +46,7 @@ public static HtmlString RenderDocTypeGridEditorItem(
             {
                 dtgeModel = content,
                 dtgeViewPath = viewPath,
-                dtgePreviewViewPath = previewViewPath,
-                dtgePreview = isPreview
+                dtgePreviewViewPath = previewViewPath
             };
 
             // Try looking for surface controller with action named after the editor alias
@@ -86,8 +84,7 @@ public static HtmlString RenderDocTypeGridEditorItem(
             }
 
             // Check for preview view
-            if (string.IsNullOrWhiteSpace(previewViewPath) == false
-                && isPreview)
+            if (string.IsNullOrWhiteSpace(previewViewPath) == false && isPreview)
             {
                 var fullPreviewViewPath = $"{previewViewPath}{editorAlias}.cshtml";
                 if (ViewHelper.ViewExists(helper.ViewContext, fullPreviewViewPath, true))