Scope tablet classes to tablets range only

Fixes #9128
Merge branch '3.0.0-wip' of github.com:twbs/bootstrap into 3.0.0-wip
Fixes #9150: Hide popover titles in IE8

Properly hides popover titles in IE8 because apparently it doesn't
accept the `:empty` selector. /cc @fat @cvrebert
Fixes #9145: Fix tooltip opacity

Update tooltips to not use RGBa colors so IE8 can have tooltips, too
(means we're now using `opacity`).