Responsive tabs display bug in IE11

For the tabs, we use display: inline-block;
https://github.com/phpbb/phpbb/blob/develop-ascraeus/phpBB/styles/prosilver/theme/cp.css#L113

The problem with this is that jQuery .show() has the following property:
"the display property is restored to whatever it was initially. If an element has a display value of inline, then is hidden and shown, it will once again be displayed inline."
http://api.jquery.com/show/

We don't want inline-block inside the responsive dropdown menu. All other browsers seem to use "block", but only IE11 apparently does the correct thing: revert to "inline-block".

So either we need to use "block" as the initial state, or don't use any display property (which will make the <li> revert to list-item.

Both seem to work.