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