This change make our test harness skip 13 tests for foreign-content
fragment parsing that have been failing from some time now. They’re
not regressions — or not recent regressions at least — so, we can
essentially treat them as "known to fail" for the time being.

This change, in order to support the "customizable" <select> element:
 - "relaxes" parsing rules in <select> (to allow more elements as children)
 - adds the <selectedcontent> element for cloning selected <option> content
ElementName.java
----------------
 - Add <selectedcontent> element with TreeBuilder.SELECTEDCONTENT group
TreeBuilder.java
----------------
 - Add SELECTEDCONTENT group constant and IN_SELECT_IN_CONTENT mode
 - Track selectedContentPointer and activeOptionStackPos for cloning
 - Allow <div>, <span>, and other elements in <select> in "relaxed" mode
 - Deep clone <option> content to <selectedcontent> when <option> closes
 - Handle <selectedcontent> special-casing (store pointer, no stack push)
SAXTreeBuilder.java
-------------------
 - Implement clearSelectedContentChildren() for clearing before clone
 - Implement deepCloneChildren() and deepCloneNode() for <option> cloning
ParentNode.java
---------------
 - Add clearChildren() method, to support clearing <selectedcontent>
html5lib-tests submodule
------------------------
 - Updated to pull in corresponding tests for "relaxed" <select> parsing