Factor out code to discard the current_folder

Factor out code to discard the current_folder
Patch by Carlos Garnacho <carlos@imendio.com>:

* gtk/gtkfilechooserentry.c (discard_current_folder): New
function, factored out for when we need to get rid of the
current_folder.
(gtk_file_chooser_entry_dispose): Use discard_current_folder().
(finished_loading_cb): Fix prototype.
(load_directory_get_folder_callback): Discard the completion
store, as well as clearing the completion feedback, if we find an
error while loading the folder.  Also, use
discard_current_folder().
(reload_current_folder): Use discard_current_folder().

Signed-off-by: Federico Mena Quintero <federico@novell.com>
svn path=/trunk/; revision=22155