Bug 234485:  Map xml:lang attribute into style so that it's used for font selection and hyphenation.  r=bzbarsky

The code in nsHTMLStyleSheet implements LangRule to map xml:lang into
style and the code to manage its uniqueness.

The change to nsGenericHTMLElement fixes the mapping of the HTML lang
attribute to do cascading the way all other rule mapping does so that
the cascading works correctly.

The tests test that the correct style language is used for hyphenation
by copying over a set of hyphenation reftests that check its basic
response to languages.  There are no specific tests for font selection,
but font selection is known to use the same language data from style.

I verified manually (see other attachments to bug) that the rule
uniqueness is being managed correctly.

--HG--
rename : layout/reftests/text/auto-hyphenation-1.html => layout/reftests/text/auto-hyphenation-xmllang-1.xhtml
rename : layout/reftests/text/auto-hyphenation-10.html => layout/reftests/text/auto-hyphenation-xmllang-10.xhtml
rename : layout/reftests/text/auto-hyphenation-1.html => layout/reftests/text/auto-hyphenation-xmllang-11a.xhtml
rename : layout/reftests/text/auto-hyphenation-1.html => layout/reftests/text/auto-hyphenation-xmllang-11b.xhtml
rename : layout/reftests/text/auto-hyphenation-4.html => layout/reftests/text/auto-hyphenation-xmllang-12a.xhtml
rename : layout/reftests/text/auto-hyphenation-4.html => layout/reftests/text/auto-hyphenation-xmllang-12b.xhtml
rename : layout/reftests/text/auto-hyphenation-1.html => layout/reftests/text/auto-hyphenation-xmllang-13a.xhtml
rename : layout/reftests/text/auto-hyphenation-1.html => layout/reftests/text/auto-hyphenation-xmllang-13b.xhtml
rename : layout/reftests/text/auto-hyphenation-4.html => layout/reftests/text/auto-hyphenation-xmllang-14a.xhtml
rename : layout/reftests/text/auto-hyphenation-4.html => layout/reftests/text/auto-hyphenation-xmllang-14b.xhtml
rename : layout/reftests/text/auto-hyphenation-1a.html => layout/reftests/text/auto-hyphenation-xmllang-1a.xhtml
rename : layout/reftests/text/auto-hyphenation-2.html => layout/reftests/text/auto-hyphenation-xmllang-2.xhtml
rename : layout/reftests/text/auto-hyphenation-3.html => layout/reftests/text/auto-hyphenation-xmllang-3.xhtml
rename : layout/reftests/text/auto-hyphenation-4.html => layout/reftests/text/auto-hyphenation-xmllang-4.xhtml
rename : layout/reftests/text/auto-hyphenation-5.html => layout/reftests/text/auto-hyphenation-xmllang-5.xhtml
rename : layout/reftests/text/auto-hyphenation-6.html => layout/reftests/text/auto-hyphenation-xmllang-6.xhtml
rename : layout/reftests/text/auto-hyphenation-7.html => layout/reftests/text/auto-hyphenation-xmllang-7.xhtml
rename : layout/reftests/text/auto-hyphenation-8.html => layout/reftests/text/auto-hyphenation-xmllang-8.xhtml
rename : layout/reftests/text/auto-hyphenation-9.html => layout/reftests/text/auto-hyphenation-xmllang-9.xhtml