Adds the new function encodings_convert_to_utf8_auto(). This makes easy
to convert file data to UTF-8 in a high-level fashion, with BOM support
and everything.

git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@5606 ea778897-0a13-0410-b9d1-a72fbfd435f5