replaced sanitize_title with sanitize_text_field

sanitize_title messes up some strings with foreign encoding/characters,
for example russian word швейцария (france) becomes
%d1%84%d1%80%d0%b0%d0%bd%d1%86%d0%b8%d1%8f
Merge branch 'trunk' of https://github.com/RistoNiinemets/qtranslate-slug into trunk