Implement :xpath syntax

Background

We should implement the :xpath syntax for element hiding. See https://github.com/gorhill/uBlock/wiki/Procedural-cosmetic-filters#subjectxpatharg

Several approaches can be taken, but the uBO syntax is more complex to implement than the snippet:

• try to convert on the fly to a SnippetFilter. This would mean adblockpluscore knows about the content of the snippets library which mean we leak the abstraction.
• implement :xpath as an element hiding emulation filter, but then the syntax isn't directly compatible.

What to change

TBD