Problem with account tree/model change with dragging (r18426)

[Phil Longstaff]

I think some problems were introduced with this change.  I created a
report and tried to modify the set of accounts used for this report.
The following showed up in /tmp/gnucash.trace:

* 20:14:20  CRIT <Gtk> gtk_tree_model_sort_convert_iter_to_child_iter:
assertion `GTK_IS_TREE_MODEL_SORT (tree_model_sort)'
failed                                                                                    
* 20:14:20  CRIT <Gtk> gtk_tree_model_sort_get_model: assertion
`GTK_IS_TREE_MODEL_SORT (tree_model)'
failed                                                                                                          
* 20:14:20  CRIT <Gtk> gtk_tree_model_filter_convert_iter_to_child_iter:
assertion `GTK_IS_TREE_MODEL_FILTER (filter)'
failed                                                                                         
* 20:14:20  CRIT <gnc.gui> gnc_tree_view_account_filter_by_view_info:
assertion `GNC_IS_ACCOUNT(acct)'
failed                                                                                                         
* 20:14:20  CRIT <Gtk> gtk_tree_model_sort_convert_iter_to_child_iter:
assertion `GTK_IS_TREE_MODEL_SORT (tree_model_sort)'
failed                                                                                    
* 20:14:20  CRIT <Gtk> gtk_tree_model_sort_get_model: assertion
`GTK_IS_TREE_MODEL_SORT (tree_model)'
failed                                                                                                          
* 20:14:20  CRIT <Gtk> gtk_tree_model_filter_convert_iter_to_child_iter:
assertion `GTK_IS_TREE_MODEL_FILTER (filter)'
failed                                                                                         
* 20:14:20  CRIT <gnc.gui> gnc_tree_view_account_filter_by_view_info:
assertion `GNC_IS_ACCOUNT(acct)'
failed                                                                                                         

More info: I set a breakpoint down in the logging code, and it stopped
on:

Breakpoint 3, log4glib_handler (log_domain=0x806c7c "GLib-GObject",
log_level=G_LOG_LEVEL_WARNING, 
    message=0xa878208 "invalid cast from `GncTreeModelAccountDrag' to
`GtkTreeModelSort'",         
    user_data=0x8052c30) at
qoflog.c:112                                                           

which may provide some clue to the problem.

Phil