[core] unify UTF-8/16 conversion to <boost/locale/encoding_utf.hpp>

platform/qt/src/utf.cpp

@@ -5,7 +5,7 @@
 namespace mbgl {
 namespace util {
 
-std::u16string utf8_to_utf16::convert(std::string const& utf8) {
+std::u16string convertUTF8ToUTF16(std::string const& utf8) {
     auto utf16 = QString::fromUtf8(utf8.data(), utf8.length());
 
     // Newers Qt have QString::toStdU16String(), but this is how it is