Simplify setMainWidget

Summary:
There's no need to create a layout for the central widget
that will hold only one widget.

Test Plan: compiled, triggered a few actions that could cause a layout change.

Reviewers: Konsole, hindenburg, ngraham

Reviewed By: ngraham

Subscribers: ngraham, konsole-devel

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D14198