gtk_file_selection_get_filename after all, as that would
cause memory leaks in all apps that use it unless they
specifically checked fot the GTk+ version.

gtk_file_selection_get_filename returns the filename in the C
runtime encoding. It calls g_filename_from_utf8, but copies the
returned string to a static buffer, which is returned. I think
this is better than returning the result from g_filename_from_utf8
directly, which would mean all apps that use it would have to free
the return value. Or should this function care about this issue at
all? Maybe a new function with clearly defined semantics.