Implement a DeeDirectoryModel

It would be nice to have a DeeModel implementation that was automagically populated (and updated) reflecting the contents of a designated directory. The columns of the model would be implicitly set by specifying a set of GIO file attributes to look up on the files in the directory.

I am thinking two constructors:

  /* varargs is a list of 'const gchar*' with gfile attributes */
  dee_directory_model_new (GFile *directory, ...);

  /* attribs is a NULL terminated array of strings with GFile attributes*/
  dee_directory_model_new_for_attribs (GFile *directory, const gchar **attribs)

FIXME: This may all break down if there's no generic way to look up the GFileAttributeType of some file attribute

Combined with a DeeTreeIndex this would make the Downloads section of the files lens trivial - and in fact if someone wanted to write a whole file browsing lens then that would be really easy as well.