Wire up cancel button in "Create PR" flow.

This fixes the first part of #325. Clicking "Create New" again after
cancel still doesn't work.

Author: grokys

src/GitHub.App/SampleData/SampleViewModels.cs

@@ -25,6 +25,7 @@ namespace GitHub.SampleData
     public class BaseViewModelDesigner : ReactiveObject, IViewModel
     {
         public ICommand Cancel { get; set; }
+        public ReactiveCommand<object> CancelCommand { get; protected set; }
         public bool IsShowing { get; set; }
         public string Title { get; set; }
 

src/GitHub.App/ViewModels/PullRequestCreationViewModel.cs

@@ -7,12 +7,17 @@
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
+using ReactiveUI;
 
 namespace GitHub.ViewModels
 {
     [ExportViewModel(ViewType = UIViewType.PRCreation)]
     [PartCreationPolicy(CreationPolicy.NonShared)]
     class PullRequestCreationViewModel : BaseViewModel, IPullRequestCreationViewModel
     {
+        public PullRequestCreationViewModel()
+        {
+            CancelCommand = ReactiveCommand.Create();
+        }
     }
 }

src/GitHub.Exports.Reactive/ViewModels/IPullRequestCreationViewModel.cs

@@ -1,7 +1,10 @@
-namespace GitHub.ViewModels
+using ReactiveUI;
+
+namespace GitHub.ViewModels
 {
     [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Design", "CA1040:AvoidEmptyInterfaces")]
     public interface IPullRequestCreationViewModel : IViewModel
     {
+        ReactiveCommand<object> CancelCommand { get; }
     }
 }

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

@@ -298,7 +298,7 @@
                                 <ColumnDefinition Width="Auto" />
                             </Grid.ColumnDefinitions>
 
-                            <Button HorizontalAlignment="Right" Content="Cancel" Grid.Column="0" Style="{StaticResource GitHubVsButton}"/>
+                            <Button Name="cancelButton" HorizontalAlignment="Right" Content="Cancel" Grid.Column="0" Style="{StaticResource GitHubVsButton}"/>
                             <Button HorizontalAlignment="Right" Margin="6,0,0,0" Content="Create pull request" Grid.Column="1" Style="{StaticResource GitHubVsPrimaryActionButton}" />
                         </Grid>
                     </StackPanel>

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

@@ -1,4 +1,5 @@
-using GitHub.Exports;
+using System;
+using GitHub.Exports;
 using GitHub.UI;
 using GitHub.ViewModels;
 using System.ComponentModel.Composition;
@@ -20,6 +21,8 @@ public PullRequestCreationView()
 
             this.WhenActivated(d =>
             {
+                d(this.BindCommand(ViewModel, vm => vm.Cancel, v => v.cancelButton));
+                d(ViewModel.CancelCommand.Subscribe(_ => NotifyCancel()));
             });
         }