Added top toolbar to the SubtitleEditor

Currently it contains only combo box wit encoding lists.
TODO: on encoding change, encode and decode subtitles and update them in DataModel
TODO: save state of widgets inside a tab as long as file connected to it is opened in FileList. Tab
      might be reinitialized several times but information about it should be stored somewhere
      (e.g. on a multi-column file list in FileList widget)