refactor: re-implement git stash apply

* supports `--index` option
* add an option to drop selected stash after applying
* remove `Pop` context menu for stash

Signed-off-by: leo <longshuang@msn.cn>

src/ViewModels/ApplyStash.cs

@@ -0,0 +1,48 @@
+using System.Threading.Tasks;
+
+namespace SourceGit.ViewModels
+{
+    public class ApplyStash : Popup
+    {
+        public Models.Stash Stash
+        {
+            get;
+            private set;
+        }
+
+        public bool RestoreIndex
+        {
+            get;
+            set;
+        } = true;
+
+        public bool DropAfterApply
+        {
+            get;
+            set;
+        } = false;
+
+        public ApplyStash(string repo, Models.Stash stash)
+        {
+            _repo = repo;
+            Stash = stash;
+            View = new Views.ApplyStash() { DataContext = this };
+        }
+
+        public override Task<bool> Sure()
+        {
+            ProgressDescription = $"Applying stash: {Stash.Name}";
+
+            return Task.Run(() =>
+            {
+                var succ = new Commands.Stash(_repo).Apply(Stash.Name, RestoreIndex);
+                if (succ && DropAfterApply)
+                    new Commands.Stash(_repo).Drop(Stash.Name);
+
+                return true;
+            });
+        }
+
+        private readonly string _repo;
+    }
+}