Support completion of attribute values in CSS mode

Support completion of attribute values in CSS mode

* lisp/textmodes/css-mode.el (css-property-alist): New defconst
holding CSS identifiers and the values they can have.
(css-property-ids): Compute dynamically from `css-property-alist'.
(css-value-class-alist): New defconst holding property value classes
and their values.
(css--property-value-cache): New variable providing a cache for
`css--property-values'.
(css--value-class-lookup): New function for computing a list of values
in a value class.
(css--property-values): New function for computing a list of possible
values for a CSS property.
(css--complete-property-value): New function for completing a property
value.
(css-completion-at-point): Add support for completing property values.
* test/lisp/textmodes/css-mode-tests.el: New file.