feature<Remote>: add 'Prune' context menu for remote to prune dead branches without fetching

src/Views/Popups/Prune.xaml

@@ -0,0 +1,10 @@
+<controls:PopupWidget 
+    x:Class="SourceGit.Views.Popups.Prune"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
+    xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
+    xmlns:controls="clr-namespace:SourceGit.Views.Controls"
+    mc:Ignorable="d" 
+    d:DesignWidth="500" Height="100">
+</controls:PopupWidget>

src/Views/Popups/Prune.xaml.cs

@@ -0,0 +1,30 @@
+using System.Threading.Tasks;
+
+namespace SourceGit.Views.Popups {
+    /// <summary>
+    ///     清理远程已删除分支
+    /// </summary>
+    public partial class Prune : Controls.PopupWidget {
+        private string repo = null;
+        private string remote = null;
+
+        public Prune(string repo, string remote) {
+            this.repo = repo;
+            this.remote = remote;
+            InitializeComponent();
+        }
+
+        public override string GetTitle() {
+            return App.Text("RemoteCM.Prune");
+        }
+
+        public override Task<bool> Start() {
+            return Task.Run(() => {
+                Models.Watcher.SetEnabled(repo, false);
+                var succ = new Commands.Remote(repo).Prune(remote);
+                Models.Watcher.SetEnabled(repo, true);
+                return succ;
+            });
+        }
+    }
+}