[gollem] backend configurable header displays

[Liam Hoekenga]

I've written a patch that will (should?) allow you to configure
which columns Gollem displays in the file manager on a per backend basis. 
Useful if you're using a VFS backend that doesn't conform to the attributes used
to describe a traditional UNIX file system, or simply if you just don't want to
display one of the columns.

    http://www.umich.edu/~liamr/horde/gollem_headers.diff

The code's inspired by Turba - eg the ability there to define which columns get
desplayed depending on datasource. It's configured in gollem/prefs.php:

// columns to be displayed
$_prefs['columns'] = array(
    'value' => "ftp\ttype\tname\tdownload\tmodified\tsize\tpermission\towner\tgr
oup",                      
    'locked' => false,
    'shared' => false,
    'type' => 'implicit'
);                      

and like turba, the first element of the value line is the name you've created
for a specific data source, not all datasources of that VFS type.

It hasn't been thoroughly tested, but is working on our installation.

Liam