1. open an existing file
2. open a new (non existant) file
3. save to an existing file
4. save to a new file
1, 3 and 4 is handeled by GtkFileChooser, but 2 is done poorly, it
requires hitting Ctrl-L and getting another dialog that has its own
problems.
I don't think (2) can be implemented at all. It is guaranteed that the
filename returned from a file-chooser in OPEN mode points to an
existing file. This could probably be handled by extending the API to
allow for a new mode (GTK_FILE_CHOOSER_ACTION_SELECT). Please submit a
bug report for this API addition.