Only update PR list state on main thread.

`IVirtualizingListSource.IsLoading` can be signalled on a background thread. This was causing a deadlock in #1828. Make sure we call `IssueListViewModelBase.UpdateState` on the UI thread.

Fixes #1828

Author: grokys

src/GitHub.App/ViewModels/GitHubPane/IssueListViewModelBase.cs

@@ -7,6 +7,7 @@
 using System.Reactive.Disposables;
 using System.Reactive.Linq;
 using System.Threading.Tasks;
+using System.Windows.Threading;
 using GitHub.Collections;
 using GitHub.Extensions;
 using GitHub.Extensions.Reactive;
@@ -209,6 +210,7 @@ public override Task Refresh()
                     this.WhenAnyValue(x => x.SelectedState),
                     this.WhenAnyValue(x => x.AuthorFilter.Selected),
                     (loading, count, _, __, ___) => Tuple.Create(loading, count))
+                .ObserveOn(RxApp.MainThreadScheduler)
                 .Subscribe(x => UpdateState(x.Item1, x.Item2)));
             dispose.Add(
                 Observable.FromEventPattern<ErrorEventArgs>(