Use wxString methods for UTF-8 conversions in wxX11 code.

Use wxString methods for UTF-8 conversions in wxX11 code.

Use wxString::utf8_str() and FromUTF8() to convert to and from UTF-8. This is
more efficient than using wxConvUTF8 as the code did before as it avoids the
conversion entirely when wxString uses UTF-8 internally (as it does by default
in wxX11) and also has the advantage of compiling in STL build unlike the old
code.

Closes #12518.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65657 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775