Responsive tabs for Seven: change the arrows to an ellipsis and an "x" close icon.

As a follow-up to #671...

When only one tab is shown (on mobile), having a downwards-pointing arrow when the rest of the tabs are expanded simply does not make any sense to me. Especially in the cases where the active tab is not the first one and we have tabs expanding upwards as well (the ones before the active tab). Can we please change that to an "x" close icon perhaps?

Also having the ellipsis changed to a left-pointing arrow when only one tab is shown does not make sense to me. Can we please change that to an ellipsis for consistency?

...so to summarize: ellipsis for all cases when there are more tabs to be expanded and an "x" close icon when the tabs are expanded (to convey the message that clicking there will actually close the expanded tabs).