refactor(directives): simplify some code

This change was part of #440 but didn't make it into master.

However I would like to propose this change:
- `cssUrl` and `cssUrls` should be protected , ie `_` prefixed
- drop the `allCssUrls` getter,
- add a `cssUrls` getter (which would be the legacy `allCssUrls`).

That would be much cleaner but a BC break (easy to fix w/ find & replace).

Thoughts ?