kernel32: Support UTF-7 in MultiByteToWideChar.

Portions of utf7_mbstowcs were written by Sebastian Lackner
<sebastian@fds-team.de>
kernel32: Support UTF-7 in WideCharToMultiByte.

Portions of utf7_wcstombs were written by Sebastian Lackner
<sebastian@fds-team.de>
kernel32/tests: Add tests for UTF-7 conversion.