Secures all ITasks, adds unit tests to ensure they are all secured and the correct app is assigned.

src/Umbraco.Web/UI/IAssignedApp.cs

@@ -0,0 +1,17 @@
+namespace Umbraco.Web.UI
+{
+    /// <summary>
+    /// This is used for anything that is assigned to an app
+    /// </summary>
+    /// <remarks>
+    /// Currently things that need to be assigned to an app in order for user security to work are:
+    /// dialogs, ITasks, editors
+    /// </remarks>
+    public interface IAssignedApp
+    {
+        /// <summary>
+        /// Returns the app alias that this element belongs to
+        /// </summary>
+        string AssignedApp { get; }
+    }
+}