Please describe the problem:
gmdsetup does not handle GtkFileChooserButton and their corresponding checkbuttons (mainly Local/Logo, Remote/Logo, Users/DefaultFace) when file is defined but is not accessible on the filesystem (this does not apply when the config entry is an empty string)

Steps to reproduce:
1. point greeter/Logo (custom.conf) to a file that does not exist
2. update the daemon's config through UPDATE_CONFIG command
3. run gdmsetup and navigate to the Local tab (gdmlogin greeter)


Actual results:
Logo/Image checkbutton is active, and the corresponding filechooser widget is enabled but the filename string displayed is None

Expected results:
Logo/Image checkbutton is deactivated, and the corresponding filechooser widget is disabled (with None string displayed)

Does this happen every time?
Yes

Other information:
Before the widget is activated the only check that is performed is
if (!ve_string_empty (logo_filename)) 
however this only works if string is empty
what we should have is
if (!ve_string_empty (logo_filename) && access (logo_filename, R_OK) == 0)

The fix is included in the next gdmsetup patch