We used to have two different models for displaying file system contents:
the FileManager-grade table-like directory model, which exposed rich data
(such as file icons with integrated image previews) about contents of a
single directory, and the tree-like GFileSystemModel, which only exposed
a tree of file names with very basic info about them.
This commit unifies the two. The new GFileSystemModel can be used both as a
tree-like and as a table-like model, or in fact in both ways simultaneously.
It exposes rich data about a file system subtree rooted at the given root.
The users of the two previous models are all ported to use this new model.
