Make blank_label_text configurable on a per-table basis

The app.conf "blank_label_text" directive sets default test to use for preferred labels when no text is set.

Currently this single setting is used for all types of records.Additional configuration directives should be added allowing configuration on a per-table basis with fallback to the universal setting. Format of table-specific directives should be <table>_blank_label_text. For example, "ca_objects_blank_label_text"