Support multiple filters in css filter property

This is a first pass at #615 (closed).

The Filter property now accepts either none or a list of filter URIs to apply to an element. FilterValue and FilterValueList are introduced to store these filters.

FilterValueList::is_applicable is introduced to check if all the URIs in the list are valid (that is, they point to an element and that element is a <filter>); the entire list is applied only if every item in it is valid.

In the future, FilterValue can grow a new option to handle filter functions like blur, etc. For now, we only parse filter URIs so providing a function will cause parsing to fail; the corresponding branch in is_applicable for functions is unreachable!().

Finally, we now iterate over each filter in the list, applying it to the output of the previous when rendering.