Added helper functions for converting between wchar and utf-8. Patch provided by Hendu.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4922 dfc29bdd-3216-0410-991c-e03cc46cb475
Updated changes.txt.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4923 dfc29bdd-3216-0410-991c-e03cc46cb475