Filechooser - In OPEN or SAVE mode, typing a plain folder name should switch to it

Filechooser - In OPEN or SAVE mode, typing a plain folder name should switch to it

If you type 'subfolder/' and hit Enter, the file chooser will switch you to
it.  But if you type just 'subfolder' without the trailing slash, apps would
receive *that* filename (e.g. file:///foo/bar/subfolder) as the response, and
they would likely print an error that they can't open that file (as it is
a folder).  So, we change the file chooser's current folder to that subfolder
in this case.

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