Simplifying GtkFileSystemModel

[Federico Mena Quintero <federico ximian com>]

GtkFileSystemModel has a bunch of delicate code to handle a multi-level
file system tree:  fancy delayed destruction of nodes, blah blah.
However, these days we never use GtkFileSystemModel as anything other
than a single-level display of a file system folder; we don't have
expandable nodes any more, as we always set the max_depth to 0.

It seems to me that we could remove all the code to handle multi-level
trees and just keep GtkFileSystemModel as simple glue between
GtkFileFolder and GtkTreeView.  This would make the code a lot smaller
and simpler to the point of being almost trivial.

[I hate to throw away so much fancy code, but it will always live in CVS
anyway :) ]

Thoughts?

  Federico