Code cleanliness fixes.

Improve code cleanliness to avoid future bugs.

Data.CSS.Style.Selector.Index: fix potential crash.

Code cleanliness: Interpret Data.CSS.Style.Importance.splitProperties return value correctly.

Factorize out cascade logic.

Add method to override CSS inheritance.

Parse HTML 'style' attributes.

Fix parser freezes, blocking issue for Rhapsode.

Fix incorrect handling of invalid syntaxes.

Ignore invalid selectors.

Aid attribute sorting, fix install of internal modules.

Test cascade logic.

Test/fix cascade logic for selector specificity.

Switch to new namespace for consistency with other modules like css-syntax.

Draft alternate API.