bgo#586315 - gtk_file_chooser_list_shortcut_folders() was crashing

bgo#586315 - gtk_file_chooser_list_shortcut_folders() was crashing

The virtual method list_shortcut_folders returns a GSList * of GFile *.
In turn, gtk_file_chooser_list_shortcut_folders() converts those to strings.

However, the delegate in gtkfilechooserutils.c was calling
gtk_file_chooser_list_shortcut_folders() every time, so we were trying
to convert invalid data.

Now we have an internal function that deals with GFile *.  That
function is called by the delegate, and the conversion is done only
once by the API entry points.

Signed-off-by: Federico Mena Quintero <federico@novell.com>