docs: add missing "Since: 3.14"
Add gtk_source_encoding_remove_duplicates()

A convenience function.
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.