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

Reviewers: VDG, mart, hindenburg

Reviewed By: hindenburg

Subscribers: ahmadsamir, romangg, ngraham, Konsole

Tags: Konsole

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