Implement (parts of) Selectors Level 4

[wosc]

Would be nice if has(arbitrary-selector) could be supported, for example

python -m cssutils.scripts.cssparse -s 'select fieldset:has(option[value="foo"]:checked) {}'
ERROR	Selector: Unexpected CHAR. [1:27: []
ERROR	Selector: Unexpected CHAR. [1:33: =]
ERROR	Selector: Unexpected CHAR. [1:36: ]]
ERROR	Selector: Unexpected start of pseudo. [1:38: :checked]
ERROR	SelectorList: Invalid Selector: select fieldset:has(option[value="foo"]:checked)

python -m cssutils.scripts.cssparse -s 'select fieldset:has(option:checked) {}'
ERROR	Selector: Unexpected start of pseudo. [1:28: :checked]
ERROR	SelectorList: Invalid Selector: select fieldset:has(option:checked)

# this very simple variant already works
python -m cssutils.scripts.cssparse -s 'select fieldset:has(option) {}'