Release 3.13.3
Post release version bump
encoding: remove unused function

Since we don't have a gsetting in GtkSourceView, list_to_strv() is
useless.
encoding: make a private function static

The private function was used only in gtksourceencoding.c.
encoding: add private function remove_duplicates()

It'll be used later.
Ensure that UTF-8 and CURRENT are present in the candidate encodings

In the gedit translations, CURRENT was sometimes translated (and there
is a translator comment to explain that it should not be translated...).
So it's better to ensure that UTF-8 and CURRENT are present, just in
case.