Thinking about it, it is better to always convert to docstrings all

Thinking about it, it is better to always convert to docstrings all
utf8 strings, and not only if they contain encoding changes. This is
because if the output encoding was previously changed and an utf8
string is inserted in the stream, the encoding changes cannot occur.
This was not a problem until now because normal strings could not be
inserted in a odocstream, as them would have been exchanged with encoding
changes. Indeed, the SetEnc struct has only a std::string member and
outputting a std::string would be interpreted by the compiler the same
as inserting setEncoding(std::string). However, a std::string can be
inserted in an otexstream and it is better to account for this.
I wonder whether trying "os << std::string", where os is an odocstream,
should produce an error instead of actually trying to change the stream
output encoding, but this has not been a problem until now...

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@39944 a592a061-630c-0410-9148-cb99ea01b6c8