Tabs: fix IE8 .tabs-roller in .tabs-style-2 (fixes #3695)

Removed the opacity filter from the .tabs-style-2 .tabs element.  IE8
is unable to properly stack a parent and child element that both have
a filter applied.

Modern browsers that understand the opacity CSS rule will still get the
0.95 opacity.
Merge pull request #3709 from patheard/tab-overlay-ie8

Tabs: fix IE8 .tabs-roller in .tabs-style-2 (fixes #3695)