Improvements to the Custom interface.

Improvements to the Custom interface.

* cus-edit.el: Resort topmost custom groups.
(custom-buffer-sort-alphabetically): Default to t.
(customize-apropos): Use apropos-parse-pattern.
(custom-search-field): New var.
(custom-buffer-create-internal): Add custom-apropos search field.
(custom-add-parent-links): Don't display parent doc.
(custom-group-value-create): Don't sort top-level custom group.
(custom-magic-value-create): Show visibility button before option
name.

(custom-variable-state): New fun, from custom-variable-state-set.
(custom-variable-state-set): Use it.
(custom-group-value-create): Hide options with standard values
using the :hidden-states property.  Use progress reporter.

(custom-show): Simplify.
(custom-visibility): Disable images by default.
(custom-variable): New property :hidden-states.
(custom-variable-value-create): Enable images for
custom-visibility widgets.  Use :hidden-states property to
determine initial visibility.

* wid-edit.el (widget-image-find): Give images center ascent.
(visibility): Add :on-image and :off-image properties.
(widget-visibility-value-create): Use them.