Use QTabBar: drop tons of code

Summary:
this is a WIP, I don't plan to merge this yet, there are many thigns to solve.

Drop the handmade TabWidget used inside of konsole in favor of Qt's QTabWidget
This drops tons of code we manually did, but it also drops a bit of functionality.

Missing features:
- Close Button
- New Tab on double click
- Drag'n Drop
- Detach

and lots of testing.

Subscribers: konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D13882