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.

Everything besides detach should be working.
I'm now trying to finish detach.

a nice overvie:
 15 files changed, 314 insertions(+), 1609 deletions(-)

Missing features:
- Detach

and lots of testing.

Reviewers: hindenburg, #konsole, ngraham

Reviewed By: ngraham

Subscribers: ngraham, konsole-devel

Tags: #konsole

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