(Based on a discussion with Seth, Jonathan)

I think we shouldn't allow these modes in 2.6. For Save:

 * Without paths, you just have 'a name' with no indication of
   where the file is going to end up.

 * With paths, you have paths (upsetting certain people), and
   you get a unhelpfully trunctated path in many cases.

 * There's a significantly better way to set up selecting 
   a filename to save in:

          [  Entry ]
       in [ Folder ]

   or possibly:

      [    Entry     ]
      in ‘Folder’ [  ]

   This combination can't really be a single widget, because 
   it wouldn't give enough packing flexibility. SO, I think we
   should just encourage people to use an entry plus ChooserButton
   in select mode.

For Create Folder, it's an extremely rare operation, and 
most of the same considerations apply as above

We could add something here later, if we figure out a good way
to go, but I think maximum future flexiblity is if we make
these g_warning() and fall back to Open mode for 2.6.