The patch adds convenience functions for working on C++11's new char
width specific unicode strings u16string and u32string.
[ChangeLog][QtCore][QString] Added methods for convenient conversion
to and from std::u16string and std::u32string.
Change-Id: I67c082e4755c592d61daaaaa70c8867ef0b23dcb
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>