Do not hide tab bar if there are less than 2 tabs

Hiding the tab bar breaks input focus navigation with Tab: If the widget
before the tab widget has focus and the tab bar is hidden, then nothing
happens when one presses Tab. Most likely a bug in QTabWidget which
sets the tab bar as focus proxy.

Moreover, not hiding the tab bar makes it more obvious that one can
create additional tabs.

GnuPG-bug-id: 5841