Allow Custom settings to be migrated into a custom theme.

Allow Custom settings to be migrated into a custom theme.

* cus-theme.el (custom-theme--migrate-settings): New var.
(customize-create-theme): Allow editing the `user' theme.
(custom-theme-add-variable, custom-theme-add-var-1)
(custom-theme-add-face, custom-theme-add-face-1): Add a checkbox
to the front of each variable or face widget.
(custom-theme-write): Save theme settings in the correct order.
Optionally, remove saved settings from user customizations.
(custom-theme-write-variables, custom-theme-write-faces): Saved
only the checked widgets.
(customize-themes): Add a link for migrating custom settings.

* custom.el (custom-declare-theme, provide-theme): Use
custom-theme-name-valid-p.
(custom-theme-name-valid-p): Remove checks that are now
unnecessary since themes no longer obey load-path.

* cus-edit.el (custom-variable-value-create): For the simple
style, hide documentation string when hidden.