Support 'de facto' UTF16-LE strings in pdf
PDF standard does not use UTF16-LE strings (instead mandates UTF16-BE) but we can find pdf files in the wild which include LE strings.
I'm attaching such a one file: INDICE2020-03-23.pdf
you'll notice the Outline items for page 14 and 31 show garbage text (a starting 'ÿþ' in the case of Okular) and in Evince even no text at all, only the two garbage chars are shown (which are the ascii representation of the UTF16 leading BOM bytes).
In Adobe Reader and in pdf.js those outline items are shown fine, so they are supporting LE strings. Let's do the same and add support for that.
Evince issue: https://gitlab.gnome.org/GNOME/evince/-/issues/1444