Bug 893707 - [bb][refactor] Headers

Main points:
* Spliited headers.css in to 5 new files: layout.css, icons.css, skin.default.css, skin.dark.css, skin.organic.css
* Refatorized selectors and some rules re-arrangment
* Icons no longer needs extra markup
* Removed unsued images
* Removed "header with form" case as in no longer supported