Always use UTF-8 for file names passed to GTK+ functions under MSW.

Always use UTF-8 for file names passed to GTK+ functions under MSW.

When building wxGTK under MSW, always use UTF-8 as file name encoding because
GTK+ doesn't use G_FILENAME_ENCODING there.

Add a helper wxGTK_CONV_FN() macro to hide the difference between the
platforms.

Closes #14035.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71154 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775