The window of the application.
It contains a Adw.NavigationSplitView as a view which has a View.FilesView
and View.EditView inside.
It also has a instance of Model.DesktopFileModel and calls
Model.DesktopFileModel.load when constructed. If the call succeeded, it
calls View.FilesView.set_list_data to reflect load result.
Otherwise,
it shows a dialog to tell the user about the failure.