Added TryFindResource extension method.

`FindResource` can be ambiguous because it returns `AvaloniaProperty.UnsetValue` in the case of a resource not being found, which is a valid resource value. `TryFindResource` removes this ambiguity. `FindResource` has been left in for API compatibility with other frameworks.

src/Avalonia.Styling/Controls/ResourceProviderExtensions.cs

@@ -1,8 +1,6 @@
 using System;
-using System.Collections.Generic;
 using System.Reactive;
 using System.Reactive.Linq;
-using System.Text;
 
 namespace Avalonia.Controls
 {
@@ -15,6 +13,23 @@ namespace Avalonia.Controls
         /// <param name="key">The resource key.</param>
         /// <returns>The resource, or <see cref="AvaloniaProperty.UnsetValue"/> if not found.</returns>
         public static object FindResource(this IResourceNode control, string key)
+        {
+            if (control.TryFindResource(key, out var value))
+            {
+                return value;
+            }
+
+            return AvaloniaProperty.UnsetValue;
+        }
+
+        /// <summary>
+        /// Tries to the specified resource by searching up the logical tree and then global styles.
+        /// </summary>
+        /// <param name="control">The control.</param>
+        /// <param name="key">The resource key.</param>
+        /// <param name="value">On return, contains the resource if found, otherwise null.</param>
+        /// <returns>True if the resource was found; otherwise false.</returns>
+        public static bool TryFindResource(this IResourceNode control, string key, out object value)
         {
             Contract.Requires<ArgumentNullException>(control != null);
             Contract.Requires<ArgumentNullException>(key != null);
@@ -25,16 +40,17 @@ namespace Avalonia.Controls
             {
                 if (current is IResourceNode host)
                 {
-                    if (host.TryGetResource(key, out var value))
+                    if (host.TryGetResource(key, out value))
                     {
-                        return value;
+                        return true;
                     }
                 }
 
                 current = current.ResourceParent;
             }
 
-            return AvaloniaProperty.UnsetValue;
+            value = null;
+            return false;
         }
 
         public static IObservable<object> GetResourceObservable(this IResourceNode target, string key)