Implement new GtkFileSystemModel

Implement new GtkFileSystemModel

The new model is mostly API-compatible with the old model (minimal
changes were required), but is a lot faster and has a lot of very
desirable features.
- the model does no longer support a tree, just a list of files in a
  given directory
- the storage has been moved to a GArray as opposed to a tree
- no more dependency on GtkFileSystem
- columns are managed by the creator of the model, so any number of
  nodes can be added as needed. This also makes the API more similar
  to GtkListStore.
- Values are filled on demand using a function given when creating the
  model.
- The function can decide to let the model cache returned values or
  decide to be called again the next time the value is queried.
- implements GtkTreeSortable
- _gtk_file_system_model_get_value() was added to significantly speed
  up value access, which is necessary when sorting large models.