Refactor MainWindow with MVVM

- Add MVVM structure(BaseViewModel)
- Create ViewModel for MainWindow
- Refactor MainWindow.xaml to support MVVM
- Move PublicAPI implementation from MainViewModel to PublicAPIInstance

Wox/Converters/VisibilityConverter.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Windows;
+using System.Windows.Data;
+
+namespace Wox.Converters
+{
+    class VisibilityConverter : ConvertorBase<VisibilityConverter>
+    {
+        public override object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            if (value == null || value == DependencyProperty.UnsetValue)
+            {
+                return null;
+            }
+
+            return bool.Parse(value.ToString()) ? Visibility.Visible : Visibility.Collapsed;
+        }
+
+        public override object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            return null;
+        }
+    }
+
+}