css: Update and expand clearfix mixin.
css: Revise and expand text-inset mixin. Breaking.

Uses a more intensive but better-quality technique 
for text-shadows. Not hardened at all. Requires 
custom html.

- Remove unneeded inverted alternate mixin.
- Add clear color.
css: Update prefixes.
css: Sweeten transition mixin interface.
css: Other minor improvements. No real interface changes.