Improve doc of gedit_document_get_metadata()
Add candidate-encodings gsetting

https://bugzilla.gnome.org/show_bug.cgi?id=730622
EncodingsDialog: make displayed encodings not sortable

For the displayed encodings (on the right, the encodings shown-in-menu),
there'll be up and down buttons.
EncodingsDialog: show UTF-8 in the available encodings
Use candidate-encodings gsetting in ComboBox and Dialog

It adds the private function gedit_settings_get_candidate_encodings().