Refactoring visibility related events

1. remove unnecessary events from MainViewModel
2. remove usage of Obsolete api (show, hide etc)
3. fix space problem in #660
4. part of #486
5. fix up/down key bug introduced in 92b7ca6a1bafd254e39ee92812ff691906cd85c1
6. fix #678

Wox/Helper/SingletonWindowOpener.cs

@@ -10,7 +10,7 @@ namespace Wox.Helper
         {
             var window = Application.Current.Windows.OfType<Window>().FirstOrDefault(x => x.GetType() == typeof(T))
                          ?? (T)Activator.CreateInstance(typeof(T), args);
-            App.API.HideApp();
+            Application.Current.MainWindow.Hide();
             window.Show();
             window.Focus();