subject says all.  the following patch:

 * ports the gtk-bookmarks file format to GBookmarkFile, while importing the old
   semi-private format (without synchronisation: it can be added if it's deemed
   important);
 * adds a function for adding per-application shortcut folders using a single
   bookmark file:

     void
     gtk_file_chooser_add_shortcuts_from_file (GtkFileChooser *chooser,
                                               const gchar    *filename,
                                               const gchar    *translation_domain)

   the translation domain is used to localise the shortcuts, so that we can patch
   intltool to extract the bookmark title and merge it into the application .pot
   file for translators to localise;
 * adds internal API for adding a shortcut folder with a label; the public API
   has not been changed.

this patch is still a proof-of-concept: it should just show how to add per-application shortcuts from files.

future improvements: adding default shortcuts from a default location ($XDG_DATA_DIRS/places/*.xbel), using the application name/group as a filter, possibly with a mmap()able cache file which gets created using a small program like gtk-update-icon-cache.