Make "file_selection_new()" take as its second argument an

Make "file_selection_new()" take as its second argument an
Ethereal-defined indication of the action (open vs. save), regardless of
whether we're building for GTK+ >= 2.4 or not; we just ignore the
argument in pre-2.4 GTK+.

Use "file_selection_new()" rather than #if'ed code to use it or
"gtk_file_chooser_dialog_new()"  for GTK+ >= 2.4 and
"gtk_file_selection_new()" or it for pre-2.4 GTK+.

Add a "file_selection_set_current_folder()" routine that does the
appropriate thing depending on whether we're GTK+ >= 2.4 or not, and use
that rather than #if'ed code to use
"gtk_file_chooser_set_current_folder()" or
"gtk_file_selection_set_filename()".

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@10511 f5534014-38df-0310-8fa8-9805f1628bb7