Implement useful features (and corrections)

Make the following enhancements (and corrections):

• Display message on mappings page when no vocabularies or filter menus are defined.
• Use index of one for first filter menu.
• Rewrite _taxonomy_filter_menu_templates to accommodate different values of $names_only.
• Clean out mappings array when filter menu is deleted.
• Add module enable hooks to add default settings on existing filter menus.
• Reenable submit handler on advanced settings and implement 'reset to defaults'.
• Remove redirects on general settings page.