Add style attribute and importance to cascade sort

[Jym77]

Resolves #1532

With CSS cascade level 5 (compared to level 4):

• Importance reverse sorting not just for origin, but also for encapsulation context. Origin was OK to ignore since we have very few important user agent rules and they are unlikely to be temptatively overridden by author rules (and even less relying on the fact that it doesn't work). Encapsulation context, however, is required to solve Handle Shadow DOM related pseudo-classes and -elements #1533.
• Style attribute now has its own step in the cascade sorting, making it more natural to handle it there.

With this PR, both importance and style attribute are now part of the Cascade. This streamline Style building since all declarations are provided, and in decreasing cascade order. So, only parsing of the value is really required. Cascade becomes a bit more complex and, notably, elements with style attribute creates new branches in the rules tree, thus decreasing the possibility of sharing. Given that style attribute declarations have higher precedence than style sheet declarations, they should be close to the leaves and this shouldn't be a big issue.

Style.from() still handles the style attribute for elements that are not part of a Document (hence have no access to style sheet and no associated cascade). This should be mostly a use case for our own tests.

[Jym77]

!pr extract