Fix Gtk-CRITICAL warnings

[ghost]

I get the following assertion failures upon deleting a node and during some other occasions whose commonalities I could not yet find out:

(tomboy:2218): Gtk-CRITICAL **: gtk_text_buffer_apply_tag: assertion `tag->table == buffer->tag_table' failed

Although these warnings/errors(?) are ugly, they don't seem to affect the default behavior of tomboy/taskmanager. Could it be that this has to do with the fixing-stuff?