Extracted EncodingInfoDataContainerView from MainWindow.

This PR set the `DataType` property  for `EncodingInfoDataContainer` instead of using the 'Key' property, so that the `ItemTemplate` property in  `ComboBox` does not need to be set explicitly. In addition, EncodingInfoDataContainerView has been extracted from MainWindow to classify according to View or ViewModel.

 I was wondering whether @YoungjaeKim will accept this PR as I modified some code made from his commit.