Restore "Show 'new tab' button" option in Konsole Settings

This was inadvertedly removed in 19.08; re-add it even though it might
add a new string.

CCBUG: 411158
FIXED-IN: 19.08.1

CCMAIL: kde-i18n-doc@kde.org
GUI:

https://invent.kde.org/kde/konsole/merge_requests/24