Bug 1096572 - [Building Blocks] Update some RTL styles to match latest RTL spec

Changes, in keeping with the latest RTL spec, as of November 5:

* buttons.css: removes a padding that seems out of place with current design, does not have an equivalent style for LTR layouts.
* headers.css: keeps the header buttons and icon visuals in same layout as LTR.
* tabs.css: for bb-tablist: keeps the buttons laid out in same horizontal layout as LTR.