Add several "Raw" functions for setting/getting text from the document

Add several "Raw" functions for setting/getting text from the document
buffer. They behave similarly to the same methods without the "Raw" in
the name, except they don't use wxStrings.  This can save some
conversions to/from unicode and utf-8 in a Unicode build.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@33639 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775