bgo#629722 - file chooser should cope with typing a directory name and then Enter

bgo#629722 - file chooser should cope with typing a directory name and then Enter

file_exists_get_info_cb() used to let some actions fall through to
the code that checks for the file type of the selected file's parent
(e.g. to see if /foo/bar *is* indeed a folder if you type /foo/bar/baz).
However, we need to be more explicit for each file chooser action, as
in SELECT_FOLDER mode typing /foo/foldername and hitting Enter would not
give you an immediate selection, but rather the file chooser would
complain that it couldn't create that directory as it already exists.

Signed-off-by: Federico Mena Quintero <federico@novell.com>