enhance: Git LFS supports (#209)

* add a new context menu to push local LFS object to selected remote
* supports to choose remote for fetch/pull/push/lock/unlock actions
* auto select remote if there's only one remote

src/ViewModels/LFSPush.cs

@@ -0,0 +1,37 @@
+using System.Collections.Generic;
+using System.Threading.Tasks;
+
+namespace SourceGit.ViewModels
+{
+    public class LFSPush : Popup
+    {
+        public List<Models.Remote> Remotes => _repo.Remotes;
+
+        public Models.Remote SelectedRemote
+        {
+            get;
+            set;
+        }
+
+        public LFSPush(Repository repo)
+        {
+            _repo = repo;
+            SelectedRemote = _repo.Remotes[0];
+            View = new Views.LFSPush() { DataContext = this };
+        }
+
+        public override Task<bool> Sure()
+        {
+            _repo.SetWatcherEnabled(false);
+            ProgressDescription = $"Push LFS objects to remote ...";
+            return Task.Run(() =>
+            {
+                new Commands.LFS(_repo.FullPath).Push(SelectedRemote.Name, SetProgressDescription);
+                CallUIThread(() => _repo.SetWatcherEnabled(true));
+                return true;
+            });
+        }
+
+        private readonly Repository _repo = null;
+    }
+}