MainWindow: Add Frameless option

Summary:
Add option to hide the title bar and frame off by default. The
window can be managed via tilling management and
alt+mouse hold, etc. This is not a normal workflow for a normal
user for a normal application, but is indeed used for power users.

Reviewers: Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: sandsmark, tcanabrava, konsole-devel

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D17588