Fixed Tab floating in CMS (fixes #1778)

Removed inline-block from header elements which seems
to cause the issues of tabs stacking on top of each other.
Replaced header background to work on multiline scenarios
Added bottom margin to breadcrumbs so they don't look cramped
when tabs are floated below them.

See https://github.com/silverstripe/silverstripe-framework/pull/1863,
https://github.com/silverstripe/silverstripe-framework/issues/1943,
https://groups.google.com/forum/?fromgroups#!topic/silverstripe-dev/c-W-ZxDlDuA,
http://open.silverstripe.org/ticket/6882
Merge pull request #2090 from chillu/pulls/tabs-float

Fixed Tab floating in CMS (fixes #1778)