Bug 905047 - [bb][refactor] Buttons

* Splitted `buttons.css` into 4 files `layout.css`, `icons.css`, `skin.default.css`, `skin.flat.css` (was lists buttons)
* Removed `compact` case as is no longer valid (deprecated)
* Disabled states are now achieved by `rgba` color model, in this way we avoid double declarations for "dark" and "white" backgrounds.
* Removed duplicated code for icons active states.
* `bbs-name` is now used as a namespace for skins (Building Block Skin) because of obvious possible collissions with main component namespaces. This change has been applied to `headers.css` aswell.
* Removed no longer needed images (disabled states)