Make all elements use SelectorsHandler and Driver of their own

1. need to pass SelectorsHandler/Driver in constructor instead of session
Moved the escaping of the xpath locator to the NamedSelector

The NamedSelector has no valid reason to expect receiving an escaped value
for the XPath locator. Calling code has no reason to know that the locator
will be inserted in an XPath query.
The method SelectorsHandler::xpathLiteral has been moved to a new Escaper
class.
Fixes #384