Move the UTF-8 string conversion helper to their respective places of use.

rtengine/safegtk.h

@@ -5,10 +5,6 @@
 #include <glibmm.h>
 #include <giomm.h>
 
-Glib::ustring safe_filename_to_utf8 (const std::string& src);
-Glib::ustring safe_locale_to_utf8 (const std::string& src);
-std::string safe_locale_from_utf8 (const Glib::ustring& utf8_str);
-
 FILE * safe_g_fopen(const Glib::ustring& src, const gchar *mode);
 bool safe_file_test (const Glib::ustring& filename, Glib::FileTest test);