Extract xpath utils into dedicated classes

Both utility classes are coming from the work on Mink 2.0 done in https://github.com/Behat/Mink/pull/427/commits

This extraction means 2 things:
- it is easier to test the xpath manipulation logic as it is not hidden inside ``Element::findAll`` anymore
- people wanting to perform some XPath escaping in their code for cases still requiring it for Mink 2.0 (i.e. when building your own XPath query and wanting to escape a literal in it) can use the Escaper class instead of using the SelectorsHandler, making their code forward-compatible with Mink 2.0 (places were you use ``SelectorsHandler::xpathLiteral`` to escape the locator passed to the named selector are better left using the SelectorsHandler as you will detect the need to remove the escaping when upgrading to 2.0).