simplify css whitelist

Our css rules try to make a distinction between pure css21 and
css-extensions, but in practice nobody wants to limit to css21, and
the separation makes it hard to audit the rules.

This CL eliminates the distinction between css21 and css-extensions,
so that there's only a single set of rules.

I have some more changes in mind, but this is a prerequisite,
so I'm trying to keep this change relatively simple. The diff is
unfortunately hard to audit, but it's mostly just combining the
css21 and css-extensions rules, re-alphabetizing, and removing the
duplicates (which are css21 rules superseded by css-extension rules)

So some of these rules are not quite what the css3 specs say.
Later, I'll fix up individual rules to conform to css3 reality.

[MikeSamuel, 2013-08-16 04:18:56]

LGTM

https://codereview.appspot.com/12899043/diff/1/src/com/google/caja/lang/css/c...
File src/com/google/caja/lang/css/css3-defs.json (left):

https://codereview.appspot.com/12899043/diff/1/src/com/google/caja/lang/css/c...
src/com/google/caja/lang/css/css3-defs.json:60: { "key": "background",
Ok, this was subsmued by the one from extensions.

[felix8a, 2013-08-19 22:40:25]

@r5565