There are a few things we can do:
1. Specify that GtkFileSystem::make_path() should fail if the filename
has path components. In the context of GtkFileChooser, this would
eventually bring up an error message box, very similar to the one
Nautilus uses when you rename a file and try to put a slash in the name.
2. Specify that GtkFileSystem::make_path() must escape the filename,
thus allowing any characters to appear in it, *and* then unescaping
filenames when reading them from the file system. This would make it
possible to have file names like "Report from the engineering/marketing
teams" above. If we do this, we should modify Nautilus as well.