Handle styles that are not direct children of templates correctly

[dfreedm]

Find the correct spot to add "included" styles in the template by
keeping track of which "concrete" style in the template we have
encountered and inserting the "included" styles before them.

Fixes #4975

[dfreedm]

Still investigating a few busted cases in Safari 10 and 9

[kevinpschaaf]

Can we add a test interleaving concrete styles & includes that would have been broken by the previous implementation?

[Westbrook]

I also like how this adheres more closely to the cascade I'd expect to find when applying something like:

<style include="first-shared-style">
/* styles that override some of the shared styles */
</style>
<style include="second-shared-style-with-more-overrides">
/* a fourth override */
</style>

Thanks for the quick attention to this!

[kevinpschaaf]

LGTM!