refactor: commandline parsing

* `--rebase-todo-editor` launches this app as a git `sequence.editor`
* `--rebase-message-editor` launches this app as a git `core.editor` which runs on background by reading rebasing jobs
* `--core-editor` launches this app as a git `core.editor`
* `--askpass` launches this app as a SSH askpass program

src/Views/CodeEditor.axaml.cs

@@ -0,0 +1,54 @@
+using System;
+using System.IO;
+using System.Text;
+using Avalonia.Input;
+using Avalonia.Interactivity;
+
+namespace SourceGit.Views
+{
+    public partial class CodeEditor : ChromelessWindow
+    {
+        public CodeEditor()
+        {
+            DataContext = this;
+            InitializeComponent();
+        }
+
+        public CodeEditor(string file)
+        {
+            _file = file;
+            DataContext = this;
+            InitializeComponent();
+
+            var content = File.ReadAllText(file).ReplaceLineEndings("\n");
+            var firstLineEnd = content.IndexOf('\n');
+            if (firstLineEnd == -1)
+            {
+                Editor.SubjectEditor.Text = content;
+            }
+            else
+            {
+                Editor.SubjectEditor.Text = content.Substring(0, firstLineEnd);
+                Editor.DescriptionEditor.Text = content.Substring(firstLineEnd + 1);
+            }
+        }
+
+        private void BeginMoveWindow(object sender, PointerPressedEventArgs e)
+        {
+            BeginMoveDrag(e);
+        }
+
+        private void CloseWindow(object sender, RoutedEventArgs e)
+        {
+            Environment.Exit(-1);
+        }
+
+        private void SaveAndClose(object sender, RoutedEventArgs e)
+        {
+            File.WriteAllText(_file, Editor.Text);
+            Environment.Exit(0);
+        }
+
+        private string _file = string.Empty;
+    }
+}