fixes #8477: use var for rounded image border-radius
Drop `.nav-header` for `.dropdown-header`

* Since we no longer have `.nav-list`, I've nuked the `.nav-header`
* However, dropdowns can still have section headings, so I've kept that
around with the addition of `.dropdown-header`
* Restyled it a smidge as well
Bump tooltips to use `@font-size-small` (11px to 12px)
Fixes #8441: remove `@font-size-mini`
Merge branch '3.0.0-wip' of github.com:twitter/bootstrap into 3.0.0-wip
make on collapse changes