filechooser: Add gtk_file_chooser_get_current_name()

Currently you can only set the current filename, but not
get it. It's useful to be able to get it in save
dialogs, where the user has typed the desired filename
and you are not in a real directory (recent used, for
example).

https://bugzilla.gnome.org/show_bug.cgi?id=702497

gtk/gtkfilechooserprivate.h

@@ -61,6 +61,7 @@ struct _GtkFileChooserIface
   GFile *        (*get_current_folder) 	   (GtkFileChooser    *chooser);
   void           (*set_current_name)   	   (GtkFileChooser    *chooser,
 					    const gchar       *name);
+  gchar *        (*get_current_name)       (GtkFileChooser    *chooser);
   gboolean       (*select_file)        	   (GtkFileChooser    *chooser,
 					    GFile             *file,
 					    GError           **error);