smyrna savefiledlg, on_gvprbuttonsave_clicked: fix memory leaks

smyrna savefiledlg, on_gvprbuttonsave_clicked: fix memory leaks

The `gtk_file_chooser_get_filename` docs¹ note:

  Return value
  …
  The currently selected filename, or NULL if no file is selected, or the
  selected file can’t be represented with a local filename. Free with g_free().

  The caller of the method takes ownership of the data, and is responsible for
  freeing it.

Contrary to this, `savefiledlg` was duplicating the pointed to data and then
losing the pointer `gtk_file_chooser_get_filename` returned.

The straightforward fix to this would be to retain the pointer and then `g_free`
it after copying its contents into the buffer `xbuf`. However, we can instead
refactor this function to avoid the copy altogether and simply pass the original
memory back to the caller, making this both a fix and an optimization.

On top of this, `on_gvprbuttonsave_clicked` was never calling `agxbfree` on its
local `xbuf`. By moving to a standard string, we also fix this memory leak.

¹ https://docs.gtk.org/gtk3/method.FileChooser.get_filename.html