Encodings combobox in SubtitleEditor replaced by a context menu.

Some changes that we had to do:
* Added explicit list of supported encodings (instead the one fetched from Python modules). It's
  compatible with chardet.
* DataModel forces that saved encodings are lower case.
* Because almost every class required QAction creating, ActionFactory class has been added.
* ChangeEncodingCommand was changed to ChangeData. The most important it did was subtitle changing
  anyway and with this change we clarify the interface a little bit.
* Currently only SPF editor and FileDialogs have knowledge if user has chosen encoding auto
  detection. They are responsible for executing auto detection and inserting its results into
  DataModel. User can't choose to auto detect encoding again once subtitle file is loaded (he has to
  explicitly specify encoding instead)
* '_' (gettext) moved to gui.Detail.
Moved '_' (gettext symbol) to utils.Locale. Deleted deprecated suboptparse.