Description:

The GtkFileSelection dialog does not warn users when selecting a directory where
he/she have no read permisions. 

What happened:

When a user try to select a directory where he/she have no permissions, the
dialog changes as he/she was opening an empty directory and does not warn the
user about he can not read the directory. Usually - even while the selection
field does not change - the user thinks he opened an empty directory.

Expected behaviour:

The user should know he's not opening the directory, and/or he can not read it.

How to reproduce:

* create a directory
* touch a file into
* change permissions -r dir
* open it with the file selector dialog.

Additional information:

In gtkfilesel.c (gtk_file_selection_populate) both the file and dir stores, are
cleared before to stat the directory to see if it can be opened. One possible
fix, could be to avoid to clear those models, until the directory could be
succesfully opened.

This way at least the user could know he can not enter the directory as his
clicks will have no effect. This could be anoying but less confusing than clear
the stores, leting the user to think he succesfully opened an empty directory.

An error dialog could be shown to the user - for better usability - warning him
about his lack of permissions to read the directory.