Move validation to xaml

Author: shana

src/GitHub.VisualStudio/UI/Views/PullRequestCreationView.xaml

@@ -149,7 +149,9 @@
                               DockPanel.Dock="Bottom"
                               Fill="{StaticResource GitHubWarningBrush}"
                               Icon="alert"
-                              ValidatesControl="{Binding ElementName=titleText}" />
+                              ValidatesControl="{Binding ElementName=titleText}"
+                              ReactiveValidator="{Binding TitleValidator, Mode=OneWay}"
+                              />
 
       <ui:PromptTextBox Height="100"
                         Margin="10,5"

src/GitHub.VisualStudio/UI/Views/PullRequestCreationView.xaml.cs

@@ -20,7 +20,6 @@ public PullRequestCreationView()
 
             this.WhenActivated(d =>
             {
-                d(this.OneWayBind(ViewModel, vm => vm.TitleValidator, v => v.titleValidationMessage.ReactiveValidator));
                 d(ViewModel.CancelCommand.Subscribe(_ => NotifyCancel()));
                 d(ViewModel.CreatePullRequest.Subscribe(_ => NotifyDone()));
             });