Default to show tab bar only when needed

Summary: It clutters the UI and can still be permanently enabled in settings if so desired.

Test Plan:
Konsole looks a lot cleaner out of the box now
{F5750441}

Reviewers: #vdg, mart, hindenburg

Reviewed By: hindenburg

Subscribers: ahmadsamir, romangg, ngraham, #konsole

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D11258