Use MSFTEDIT_CLASS / RICHEDIT_CLASS instead of just "RichEdit20W"

Since MFC 2012, MSFTEDIT_CLASS is used as class name of CRichEditCtrl Unicode build; Older MFC or ANSI build uses RICHEDIT_CLASS

http://msdn.microsoft.com/en-us/library/bb531344.aspx

(Fixes issue #1781)

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>