Do not show tab headers marked as hidden

Tab headers that should not be shown are marked with the "hidden" CSS
class. The CSS rules set "display: none" for ".hidden" elements, but as
the rules for ".tabHeaders .tabHeader" are more specific than rules for
".hidden" the "display" property is overriden and ends being "flex".
Therefore, it is necessary to explicitly set a rule for ".tabHeaders
.tabHeader.hidden" elements.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>